Why this guide exists
Cold email has become a required growth tactic for B2B organizations. Outbound is non-negotiable for pipeline — even product-led companies run cold motions, agencies live and die by deliverability, and the bar keeps going up as Google and Microsoft tighten enforcement.
Deliverability is one of the least understood concepts in all of GTM. It is shaped by content, sending domains, authentication, warmup, infrastructure choices, and data quality — and most teams only learn this after a single bad campaign torches a domain they were planning to use for years.
The goal of this guide is to demystify the entire stack. Whether you are thinking about getting started or you are a pro tuning a working program, the same playbook applies: get the infrastructure right, send to data you can trust, monitor a small set of KPIs religiously, and know how to read the bounce log when something breaks. Each section below is a complete, opinionated answer to one piece of that puzzle.
TL;DR
Cold email deliverability fails in predictable ways. After analyzing hundreds of campaign post-mortems, the same five root causes account for roughly 90% of all deliverability failures:
| # | Root Cause | Frequency | Quick Test |
|---|---|---|---|
| 1 | Stale or unverified list data | ~40% | |
| 2 | Broken DNS authentication (SPF/DKIM/DMARC) | ~25% | |
| 3 | Premature sending before warmup completes | ~15% | |
| 4 | Sending too fast, too soon | ~10% | |
| 5 | Spam-trigger copy or excessive links | ~10% |
The single most misunderstood factor: Catch-all domains. Between 10-20% of B2B domains use catch-all mail server configurations that make standard email verifiers return false positives. Most senders don't know this is happening, but RevenueBase Email Verification can easily tell if these emails are Valid or Invalid. List Quality & Verification explains this in full and shows how to fix it.
Three things to do right now if you're having deliverability problems:
Why Cold Email Deliverability Fails
The failure landscape and the five pain points everyone runs into — based on community discussion patterns across the cold email ecosystem
The deliverability failure landscape follows predictable patterns. Understanding these root causes is the first step to fixing them.
Cold Email Campaign Failure Modes by Root Cause
• Broken DNS records (SPF/DKIM/DMARC failing silently)
• Sending to lists with high invalid or catch-all rates
• Wrong warmup-to-cold-send volume ratios (scaling cold sends too fast relative to warmup volume)
• Warmup pool with no genuine replies (one-way sends don't build reputation)
Inbox Infrastructure Setup
Sending domains, DNS authentication (SPF/DKIM/DMARC), mailbox configuration, and warmup protocol
Critical Rule
Choosing Your Sending Domains
Domain Setup Checklist
Choose Your Email Provider
| Factor | Google Workspace | Microsoft 365 |
|---|---|---|
| Monthly cost | $6/user (Starter) | $6/user (Business Basic) |
| Daily send limit per mailbox | 2,000 | 10,000 |
| Warmup tool compatibility | Excellent | Good — growing |
| Spam filter strictness (post-2024) | More aggressive | More forgiving |
| Setup complexity | Simple | Slightly more complex |
| Best for | Teams starting out | High-volume teams (1,000+/day/mailbox) |
2025–2026 Enforcement Update
SPF Record
SPF Setup
DKIM Record
DKIM Setup
DMARC Record
DMARC Setup
Mailbox Setup
Non-negotiable Rule
Warmup Duration by Scenario
| Scenario | Recommended Duration |
|---|---|
| Brand new domain (0 days old) | 45–60 days total (14-day aging + 30-day warmup) |
| New domain (14–30 days old) | 30 days of active warmup |
| Previously used domain (no bad history) | 14–21 days |
| Domain with prior spam complaints | 45+ days — consider registering a new domain instead |
| Existing mailbox on warmed domain | 7–14 days before scaling |
Weekly Volume Ramp Schedule
Emails Per Mailbox Per Day:
Warmup Best Practices
Instantly Inbox Setup
Workspace, warmup, and campaign configuration — verified against Instantly's help center
Account Setup
Connection Setup
Warmup Configuration
Campaign Settings
Deliverability Tools
Lead Management
Smartlead Inbox Setup
Account, warmup, and campaign configuration — verified against Smartlead's help center
Account Setup
Connection Setup
Warmup Configuration
Campaign Settings
Auto-Pause Setup
Lead Management
List Quality & Verification
Your data quality is the single biggest variable in your deliverability — even the best infrastructure can't save you from sending to 20% invalid addresses
The best infrastructure in the world cannot protect you from sending to 20% invalid addresses.
Annual B2B Email Data Decay Rate
B2B contact data decays because:
- People change employers — their work email becomes invalid
- Companies are acquired, rebranded, or shut down
- Small companies let domains lapse
- Off-boarded employees' mailboxes are deleted, typically within 30-90 days
This is the most misunderstood concept in B2B email verification. Read carefully.
When a standard email verifier tries to check if john.smith@bigcorp.comis valid, it uses SMTP to knock on the mail server and ask "does this mailbox exist?" A catch-all server answers "yes" to every knock — not just for John Smith, but for every possible address at that domain.
The result: a standard verifier cannot tell if any specific email at a catch-all domain is real. So it labels the address "catch-all," "unknown," "risky," or "accept-all."These are all the same thing: the verifier failed to get an answer and is reporting its uncertainty.
The "Black Hole" Problem
The impact:
- Your bounce rate looks artificially low (no hard bounce returned)
- Your open rate drops (nobody ever received the message)
- You are burning sends and eroding your sender score simultaneously
Between 10-20% of B2B domains behave this way.
Catch-All Rate by Company Tier
Note: Accuracy figures reflect real-world cold email sending performance, not self-reported vendor claims.
| Vendor | Est. Accuracy | Catch-All Handling | Best For |
|---|---|---|---|
| RevenueBase | 95-98% | Resolves catch-alls | Enterprise-grade B2B accuracy, AI-ready data |
| Apollo | 78-87% | SMTP-based — returns catch-all label | Broad prospecting, integrated sequences |
| Clay | 88-94% | Depends on waterfall sources | Waterfall enrichment from 75+ sources |
| Cognism | 85-92% | SMTP-based | European data, phone-verified contacts |
| Fullenrich | 82-90% | Waterfall — inherits source limitations | Waterfall enrichment, good price/volume ratio |
| Hunter.io | 82-90% | SMTP-based | Domain-level email finding, SMB |
| Lusha | 80-87% | SMTP-based | European contacts, phone data |
| Tool | Method | Catch-All Handling | Best Use Case |
|---|---|---|---|
| RevenueBase | Domain-level existence check | Resolves catch-alls | Best accuracy for enterprise B2B lists |
| NeverBounce | SMTP ping | Returns "unknown" | Good bulk pricing, solid API |
| ZeroBounce | SMTP ping | Returns "catch-all" label | Strong on disposable/spam-trap detection |
| MillionVerifier | SMTP ping | Returns "risky" | Best price-to-volume ratio |
| Bouncer | SMTP ping | Returns "unknown" | Strong European compliance features |
| Debounce | SMTP ping | Returns "catch-all" label | Budget-friendly for high volume |
| Invalid Rate in Raw Vendor Output | What this tells you about your data source | Action |
|---|---|---|
| Under 2% | Excellent source | Send the Valid subset; monitor bounce rate on launch |
| 2–5% | Healthy source | Send the Valid subset; proceed normally |
| 5–10% | Mid-quality source | Send the Valid subset; consider better data for the next list |
| 10%+ | Degraded source | Send the Valid subset for this campaign; switch vendors before the next one |
| Over 15% catch-all (SMTP verifier output) | Verifier couldn't resolve these — they're not actually invalid | Run through RevenueBase first — it resolves catch-alls and converts many to Valid |
| Any spam traps detected | Source is contaminated | Do not send any of these — remove all flagged addresses and investigate the source |
Run this before every new campaign:
Pre-Campaign List Cleaning
Spam Triggers
Words and patterns that flag your email — and the B2B-safe alternatives. Infrastructure and reputation matter more than keywords, but both matter.
How Spam Filters Actually Work (2026)
This is the gate. If authentication fails, nothing else matters — your email is rejected outright or routed directly to spam regardless of what the copy says. Fix this first.
Gmail and Outlook watch how recipients interact with email from your domain over time. Low engagement trains the filter to deprioritize you. High complaint rates trigger enforcement.
Content signals matter, but they're the last thing filters check — and modern ML-based filters look for structural patterns, not keyword lists. A perfectly worded email from a domain with no warmup still lands in spam.
Spam Trigger Word Reference
These words don't automatically send your email to spam — context, volume, and your sender reputation all factor in. A cold email from a warmed domain with clean authentication can include "free" without issue. The same word from a brand-new domain with no warmup history will likely get filtered. Use this table as a guide, not a blocklist.
| Word / Phrase | Category | Risk Level | Safer Alternative |
|---|---|---|---|
| Free | Financial | HIGH | Complimentary / at no cost / included |
| FREE!!! | Financial | HIGH | (avoid) |
| Make money | Financial | HIGH | Drive revenue / generate pipeline |
| Earn extra cash | Financial | HIGH | (avoid entirely) |
| 100% free | Financial | HIGH | No cost to start |
| Cash bonus | Financial | HIGH | Incentive / reward |
| No cost | Financial | MEDIUM | Included in your plan |
| Save big | Financial | MEDIUM | Reduce costs / cut spend |
| Investment | Financial | MEDIUM | Budget / allocation |
| Profit | Financial | MEDIUM | Revenue / return |
| Earn | Financial | MEDIUM | Generate / drive |
| Double your | Financial | HIGH | Grow your / improve your |
| Act now | Urgency | HIGH | Reply by [specific date] |
| Urgent | Urgency | HIGH | Time-sensitive / by Friday |
| Limited time offer | Urgency | HIGH | Available through [date] |
| Last chance | Urgency | HIGH | Closing soon / wrapping up |
| Don't delay | Urgency | HIGH | (avoid) |
| Offer expires | Urgency | MEDIUM | Available through [date] |
| Now or never | Urgency | HIGH | (avoid) |
| Final notice | Urgency | HIGH | Following up |
| While supplies last | Urgency | MEDIUM | (avoid for B2B) |
| Hurry | Urgency | MEDIUM | (avoid) |
| Immediate action | Urgency | HIGH | Action needed by [date] |
| Guaranteed | Promises | HIGH | We've seen / typical results |
| 100% satisfied | Promises | HIGH | (avoid) |
| Risk-free | Promises | HIGH | Try it first / cancel anytime |
| No risk | Promises | HIGH | No commitment needed |
| Promise | Promises | MEDIUM | We commit to / you can expect |
| Certified | Promises | MEDIUM | Verified / validated |
| Satisfaction guaranteed | Promises | HIGH | (avoid) |
| Results guaranteed | Promises | HIGH | Teams like yours see / typical outcome |
| No questions asked | Promises | HIGH | (avoid) |
| Congratulations | Manipulation | HIGH | (avoid in cold email) |
| You've been selected | Manipulation | HIGH | (avoid) |
| You're a winner | Manipulation | HIGH | (avoid) |
| Claim your | Manipulation | HIGH | Access your / get your |
| This isn't spam | Manipulation | HIGH | (never say this) |
| Not junk | Manipulation | HIGH | (never say this) |
| Opt in | Manipulation | MEDIUM | Sign up / get access |
| Remove from list | Manipulation | MEDIUM | Unsubscribe / stop receiving |
| This is not a solicitation | Manipulation | HIGH | (avoid) |
| Click here | Identity | HIGH | [Descriptive anchor text] |
| Visit our website | Identity | MEDIUM | Specific page name as link text |
| Call now | Identity | HIGH | Book a call / let's connect |
| Buy now | Identity | HIGH | Get started / try it |
| Order now | Identity | HIGH | Get started / request access |
| Subscribe now | Identity | MEDIUM | Join / get access |
| Sign up free | Identity | MEDIUM | Create your account / get started |
| Unsubscribe | Identity | CAUTION | (required by law — keep it, just don't lead with it) |
| Dear [first name] | Identity | MEDIUM | Hi [first name] / Hey [first name] |
| To whom it may concern | Identity | HIGH | (always use a name) |
| Hello friend | Identity | HIGH | (avoid) |
Formatting Triggers
These are structural patterns that modern spam filters flag — often more reliably than individual words, because they're harder to game.
Subject: YOU WON'T BELIEVE THIS RESULT triggers aggressive ML classifiers. Any all-caps word in a subject line adds meaningful spam score.
More than one exclamation point in a subject line (Great news!!) is a reliable spam signal. Zero or one per email is the rule.
Each link is a vector for phishing. Filters count them. For cold outreach, aim for one link or zero. Your CTA should be a reply request, not a URL.
bit.ly, tinyurl.com, ow.ly, and all short link services are flagged by default. Spam filters associate them with click-tracking obfuscation. Use full, clean URLs only.
Emails that are mostly image with little text look like spam to filters (spammers put text inside images to avoid keyword scanning). Keep images below 40% of email content by area.
Never attach files to a cold outreach email. Even a PDF one-pager from a clean domain adds significant spam score on first contact from an unknown sender.
White text on white background, 1px font, opacity 0 — spam filters can read this. Any attempt to stuff keywords invisibly triggers immediate classification.
More than 2 font sizes or any non-standard text colors in HTML email is a spam signal. Plain text or simple HTML performs best for cold outreach.
HTML-only emails with no multipart plain-text alternative get lower deliverability scores from major providers. Sending platforms handle this automatically — make sure yours is configured correctly.
Required by CAN-SPAM, GDPR, and CASL. Google's bulk sender rules (active since 2024) mandate one-click unsubscribe (RFC 8058) for senders over 5,000 emails/day to Gmail addresses. Violating this triggers enforcement.
Technical Triggers
| Trigger | Why It Flags | Fix |
|---|---|---|
| Sending from a free email domain (gmail.com, yahoo.com) | Major providers immediately classify cold email from free domains as spam — no business legitimacy signal | Register a sending domain. Never cold email from a personal Gmail. |
| From name ≠ domain (e.g. From: "Apple Support" sent from randomdomain.com) | Classic phishing pattern — filters check alignment between display name and sending domain | Keep your display name consistent with your sending domain |
| Mismatched From / Reply-To domains | Suggests the sender is hiding who they are | Send and receive replies on the same domain |
| No PTR (reverse DNS) record | Legitimate mail servers have reverse DNS. Missing PTR is a strong spam signal to receiving servers | Ask your hosting provider to set a PTR record pointing to your sending domain |
| Sending from a shared IP with spam history | If your email provider puts you on a shared IP that's been abused, you inherit the reputation | Use dedicated IPs for high-volume sending, or choose providers with clean shared IP pools |
| Sudden volume spike with no warmup | Going from 0 to 500 emails/day from a new domain in one week is a textbook spam pattern | Follow the warmup ramp in the Infrastructure section of this guide |
| Re-using exact templates across thousands of sends | Modern ML filters detect duplicate content patterns even with minor personalization tokens | Vary templates. Use A/B variants. Personalize beyond first name. |
| Sending to addresses that haven't engaged in 6+ months | Low engagement on your list trains filters to deprioritize your domain across all future sends | Segment and suppress unengaged addresses. Send only to people who are likely to respond. |
Test Before Every Campaign
Run these three checks before launching any new campaign or template. Takes under 10 minutes total.
KPIs & Monitoring
KPI thresholds, provider enforcement rules, emergency response protocol, and daily / pre-launch checklists
| Metric | Healthy | Warning | Critical | Immediate Action |
|---|---|---|---|---|
| Hard Bounce Rate | < 1% | 1–2% | > 2% | Pause campaign; clean list; re-verify before resuming |
| Spam Complaint Rate | < 0.05% | 0.05–0.1% | > 0.1% | Pause immediately; review targeting and copy |
| Cold Email Open Rate | > 35% | 20–35% | < 20% | Likely in spam — run inbox placement test |
| Cold Email Reply Rate | > 3% | 1–3% | < 1% | Inbox placement or copy issue — diagnose before scaling |
| Unsubscribe Rate | < 0.5% | 0.5–2% | > 2% | List quality or relevance problem |
| Soft Bounce Rate | < 3% | 3–7% | > 7% | Reduce volume; investigate throttling patterns |
| Daily Volume / Mailbox | ≤ 120 | 120–150 | > 150 | Risk of ISP rate limits — spread across more mailboxes (matches the warmup-ramp ceiling of 120–150/day at full volume) |
| Mailboxes / Domain | 2–3 | 3–4 | > 4 | Concentrates reputation risk — add domains |
| Provider | Spam Rate Threshold | Action at Threshold |
|---|---|---|
| Google (Gmail) | Target < 0.1%; bulk-sender hard limit 0.3% | Increased spam filtering below 0.3%; rejection (550 5.7.x) once sustained above 0.3% |
| Google (Workspace) | Same bulk-sender rules apply to Workspace-sent mail | Account review; potential sending suspension if abuse confirmed |
| Microsoft (Outlook/Hotmail) | No published numeric threshold — industry guidance: alert at 0.1%, treat 0.5%+ as critical | Filtering and potential IP/domain listing surfaced in SNDS |
| Microsoft (365 Tenant) | No published numeric threshold | Outbound spam policy can trigger a tenant-level send block |
| Yahoo / AOL | Target < 0.1%; bulk-sender hard limit 0.3% | Delivery restrictions and filtering |
Campaign auto-paused; sending mailbox rate-limited
--- Emails route to Spam instead of Primary
--- Google Postmaster Tools sender score drops
Domain appears on commercial blocklists (Spamhaus, Barracuda)
--- Google Workspace account flagged for abuse review
--- Microsoft SNDS listing — affects ALL senders from your IP range
Domain reputation permanently damaged
--- Inbox placement rates drop for all future sends from the domain
--- Best practice: retire domain, register new one
--- If Google Workspace account suspended: ALL mailboxes lose access
Key Insight
Trigger Conditions
Emergency Response Steps
Daily Health Check
Launch Readiness
Email list verification (resolves catch-alls)
Check SPF / DKIM / DMARC / Blacklists
Check sender score
Inbox placement testing
Email spam score + copy check
DMARC alignment check
Monitor Google sender reputation
Monitor Microsoft reputation
Deliverability Issues
Diagnose and fix the failure modes that pause campaigns, drop open rates, or land you on a blocklist
What happened:
Your sending platform hit a configured threshold. The typical trigger is bounce rate crossing the auto-pause limit you set (3% is a common starting value in Smartlead). For spam complaints, Google's bulk-sender requirement is to stay below 0.3% — with a recommended target under 0.1% — so set your platform's complaint alert near that target. Consecutive delivery failures on multiple mailboxes can also trigger a pause.
Diagnose:
- Open the paused campaign and look at its analytics / error view — both platforms surface the pause reason thereSmartlead docsInstantly docs
- Identify whether it was triggered by bounce rate, spam rate, or delivery failure — Instantly tags this in the Bounces error message; Smartlead exposes it on the campaign Analytics tabSmartlead docsInstantly docs
- Pull the bounce report and check which addresses triggered bounces — both platforms let you filter the campaign inbox to Bounced and export to CSVSmartlead docsInstantly docs
- Check whether bounces are concentrated in one email provider (all Gmail, all Outlook, etc.) — group the exported bounce report by recipient domain
Fix:
- If bounce-rate triggered: Remove all hard-bounced addresses; re-verify list before resuming
- If spam-rate triggered: Review email copy for spam trigger words; check relevance of targeting
- If delivery failure triggered: Check DNS records on the sending domain; verify SPF/DKIM/DMARC are passing
- Run an inbox placement test before resuming — Instantly's Inbox Placement feature or any third-party tool from the toolkit
What happened:
Your sending domain or IP has been added to a commercial blocklist. The widely-used blocklists are Spamhaus (ZEN), Barracuda Reputation Block List (BRBL), SpamCop, and SORBS. URI-based lists like SURBL and URIBL list domains that appear in spam message content. MXToolbox is a checker that queries ~100 blocklists — it isn't a blocklist itself.
Diagnose:
Fix:
- Stop all sending from the blacklisted domain immediately
- Submit removal requests (each blacklist has its own process)
- Remove the root cause: clean list, fix DNS, remove spam-trigger content
- If blacklisted multiple times: retire the domain — don't try to rehabilitate it
- Allow 24–72 hours for delisting to propagate before resuming
What happened:
Emails are delivered but not to Primary inbox. Gmail routes to Spam or Promotions; Outlook sends to Junk.
Diagnose:
- Test inbox placement at glockapps.com or mail-tester.com
- Check sender score at senderscore.org — note: scores sending IPs, not domains. If you send from shared Google/Microsoft mailboxes you're seeing the provider's IP reputation, not your own
- Check Google Postmaster Tools at postmaster.google.com — this is the authoritative reputation signal for your sending domain at Gmail
- Determine if the issue is provider-specific (only Gmail? only Outlook?) or universal
Fix:
- Fix — Infrastructure: Verify SPF, DKIM, DMARC are all passing
- Check domain age — new domains carry less established reputation; aim for 30+ days of warmup history before high-volume cold sends (community heuristic; Google/Microsoft don't publish a specific age cutoff)
- Ensure warmup is active with healthy engagement
- Fix — Content: Remove spam trigger words (free, guarantee, click here, limited time offer)
- Remove all link shorteners (bit.ly, tinyurl.com) — strong spam signals
- Reduce links per email (aim for zero or one in cold outreach)
- Test without open tracking pixels — these can trigger spam filters
What happened:
Google Workspace or Microsoft 365 suspended or limited your sending account directly — not just a campaign pause in Smartlead/Instantly.
Diagnose:
- Log into the mailbox directly (Gmail/Outlook web) to check for suspension notices
- Check whether you can send manually from the mailbox
- Review Google Admin console or M365 Admin Center for account flags or security alerts
Fix:
- Google Workspace: Submit a review request via Admin console; explain you are a legitimate business
- Microsoft 365: Contact Microsoft support directly; accounts can often be reinstated with verification
- Reduce daily sending volume significantly before attempting to resume (start at 20–30/day)
- Do not log in from unfamiliar IPs — this triggers additional security flags
- If suspended repeatedly: Migrate affected domains to the other platform (Workspace <> M365)
What happened:
Bounce rate has crossed acceptable thresholds. Common causes: stale data, unverified list, domain-level rejection by a specific provider, or authentication failure.
Diagnose:
- Export the full bounce report from the campaign (CSV) and sort by SMTP error codeSmartlead docsInstantly docs
- Group bounces by provider (Gmail vs. Outlook vs. corporate) and by code type (5xx vs. 4xx)
- 550 5.1.1 codes concentrated: data problem — addresses are invalid
- 550 5.7.x codes concentrated: authentication or policy problem
- 421 / 4xx codes concentrated: temporary throttling
Fix:
- Invalid addresses (5.1.1, 5.1.2): Remove; re-verify entire list before continuing
- Policy rejection (5.7.x): Fix authentication records; check if IP/domain is blacklisted
- Throttling (4xx): Reduce daily sending volume; spread sends over longer windows
- If bounce rate stays above 5% AFTER running through RevenueBase verification, the data source is so degraded that even verification can't rescue it — switch vendors before the next campaign. RevenueBase removes Invalids by definition, so a high post-verification bounce rate points at the source, not your sending stack.
What happened:
Campaigns are sending but nobody is replying.
Diagnose:
- Run an inbox placement test immediately — confirm emails are reaching Primary inboxes (Instantly's built-in Inbox Placement is the easiest path; or use GlockApps / mail-tester.com)Instantly docs
- Check open rate on the campaign Analytics tab — if under 20%, emails are likely going to spamSmartlead docsInstantly docs
- If open rate is normal (30–50%) but reply rate is low (under 2%), this is a copy or targeting issue, not a deliverability issue
Fix:
- If open rate is low: Focus on deliverability fixes from Failure Mode #3
- If open rate is normal: Review email copy and targeting strategy
What happened:
Your email failed DMARC alignment. The receiving server is rejecting because SPF or DKIM is not passing in alignment with your DMARC policy.
Diagnose:
Fix:
- Temporarily set DMARC to p=none to diagnose without blocking email
- Fix alignment issues in SPF and DKIM
- Re-tighten DMARC policy after alignment is confirmed
What happened:
Your bounce report shows soft bounces or missing engagement from a significant portion of your list, but few hard 5xx codes. This is the classic catch-all "black hole" pattern.
Diagnose:
- Export the campaign bounce report and filter for soft bounces or "no delivery record" entries — both platforms support filter + CSV exportSmartlead docsInstantly docs
- Check if affected domains are large corporate domains (Fortune 500, healthcare, finance)
- Run affected addresses through RevenueBase verification — it will return Valid/Invalid/Unknown without the SMTP catch-all problem
Note
Bounce Codes
Complete SMTP reference for every bounce code — what it means and what to do
Every failed email in your sending platform logs a bounce code. This section is the complete reference for what each code means, whether it is a data problem, infrastructure problem, or content/policy problem — and what to do.
| Code Type | What It Means | Auto-Retry? |
|---|---|---|
| 4xx (Soft Bounce) | Temporary failure — receiving server says "not now, try later." Your mail server retries automatically over 24–72 hours. | Yes — auto-retry |
| 5xx (Hard Bounce) | Permanent failure — receiving server says "no, never." Retrying produces the same result. Suppress the address. | No — suppress immediately |
| Category | Type | What It Means |
|---|---|---|
| DATA | Hard bounce | Invalid or nonexistent email address |
| INFRA | Hard bounce | Your DNS, authentication, or IP reputation |
| POLICY | Hard or soft | Recipient server content/spam policy |
| TEMP | Soft bounce | Temporary condition — retry automatically |
Major providers append human-readable strings to standard codes. These tell you exactly what triggered rejection:
| Diagnostic String | Provider | What It Means |
|---|---|---|
| "Your message was blocked because the sender is unauthenticated" | DMARC failure — fix SPF/DKIM alignment | |
| "Our system has detected that this message is likely unsolicited mail" | Sending reputation is low or content triggered spam classifiers | |
| "The email account that you tried to reach does not exist" | Invalid address — remove from list | |
| "Mail rejected by Windows Live Hotmail for policy reasons" | Microsoft | IP or domain on Microsoft's SNDS blocklist |
| "Message rejected due to sender's blocked status" | Microsoft | IP is on Microsoft's blocklist — submit JMRP/SNDS removal |
| "Spam message rejected" | Barracuda | Recipient's Barracuda gateway blocked it — ask recipient to whitelist |
| "This message was classified as spam" | SpamAssassin | Content-based filter triggered — remove spam trigger words |
Pro Tip
26 Free Email Deliverability Tools
Check SPF, DKIM, DMARC records and diagnose DNS issues
Check if your domain is on any email blacklists
The only verifier that resolves catch-all domains accurately
Test your email spam score before sending campaigns
Check your IP reputation and sender score
Test inbox placement across multiple email providers
Analyze and validate your DMARC configuration
Monitor Gmail deliverability and reputation metrics
Smart Network Data Services for Outlook/Hotmail deliverability
Google's own MX, SPF, and DKIM record checker. The most authoritative tool for diagnosing why Gmail is rejecting your mail — run this first when Google starts hard-bouncing your sends.
Paste raw email headers and get a decoded breakdown of the full delivery path, authentication results (SPF, DKIM, DMARC), and each server hop with timestamps.
Test your SMTP server's connectivity, TLS support, and response codes directly from MXToolbox's servers. Useful for confirming your mail server is reachable before connecting it to Smartlead or Instantly.
Upload an EML file or paste raw headers for a visual authentication chain breakdown — SPF pass/fail, DKIM signature validation, and DMARC alignment shown in plain English with color-coded results.
Check whether your SPF, DKIM, or DMARC record changes have propagated globally. Shows results from 100+ DNS servers worldwide — critical after any DNS change.
Check your sending domain or IP reputation against Cisco's threat intelligence network — one of the most widely deployed email security systems in enterprise environments.
Look up your IP address in Barracuda's reputation database — one of the most common enterprise spam filtering systems. If you're bouncing at mid-market and enterprise companies, start here.
The most authoritative spam blocklist in the world. Check whether your sending IP or domain appears on SBL, XBL, PBL, or DBL — the four Spamhaus lists that every major mail provider queries.
Check your sending IP against 300+ real-time blacklists simultaneously — more comprehensive than MXToolbox's blacklist checker. Run this when you suspect a deliverability problem.
IP reputation check against 80+ blacklisting engines including Spamhaus, Barracuda, PhishTank, ThreatLog, and SURBL. Useful for checking shared or dedicated IPs before launch.
Paste your raw email HTML and get a SpamAssassin score with every failing rule broken out individually. Shows exactly which content patterns are triggering the spam filter.
Send a test email to a provided address and get a deliverability report covering SPF/DKIM/DMARC authentication, blacklist status, content spam score, and header analysis.
Send a test email to their address and get a pass/fail report on SPF, DKIM, DomainKeys, SpamAssassin score, and Razor2. Simple and fast for a quick gut-check.
Free inbox placement test that sends your email to real Gmail and Outlook seed accounts and reports where it landed — Primary, Promotions, or Spam.
Email testing sandbox that catches outgoing emails without delivering to real recipients. Free plan includes 1,000 test emails/month with full header and spam score analysis.
Test what data your email client leaks when you open an email — IP address, device type, email client version. Useful for understanding what tracking pixels reveal.
Yahoo and AOL's sender portal — the equivalent of Google Postmaster Tools for Yahoo-hosted email. Monitor your sending reputation and request removal from Yahoo's blocklist.