Email Deliverability Troubleshooting Guide

When emails don't reach the inbox, the causes aren't always obvious. Deliverability problems can stem from technical issues, reputation damage, content triggers, or list quality problems. This troubleshooting guide helps you systematically diagnose issues and implement effective fixes.

Understanding Deliverability Problems

Recognizing when something's wrong.

Warning Signs

Metric-Based Signals:

• Open rates dropping suddenly
• Open rates declining gradually over time
• Bounce rates increasing
• Complaint rates rising
• Click rates falling without content changes

Feedback Signals:

• Customers saying they didn't receive emails
• Emails found in spam folders
• Support tickets about missing communications
• Delivery confirmation without engagement

Technical Signals:

• Bounce messages increasing
• Blocklist notifications
• Authentication failures in logs
• ISP feedback loop complaints

Establishing Baselines

Know Your Normal: Before troubleshooting, know what normal looks like:

• Typical open rate range
• Expected bounce rate
• Normal complaint rate
• Usual delivery times

Deviation Detection: Problems are changes from baseline:

• 20%+ drop in open rates = investigate
• Bounce rate over 2% = concern
• Complaint rate over 0.1% = urgent

Diagnostic Framework

Systematic approach to finding problems.

Step 1: Gather Data

Collect Information:

• Current metrics (opens, clicks, bounces, complaints)
• Historical comparison (week over week, month over month)
• Affected campaigns (all or specific)
• Affected segments (all or specific)
• Affected ISPs (Gmail, Outlook, Yahoo, etc.)

Questions to Answer:

1. When did the problem start?
2. What changed around that time?
3. Is it affecting all emails or some?
4. Is it affecting all recipients or segments?
5. Is it affecting all ISPs or specific ones?

Step 2: Categorize the Problem

Problem Categories:

Step 3: Isolate Variables

Testing Approach:

• Send to small test segment
• Try different content
• Test different sending times
• Check specific ISPs separately
• Verify technical setup

Authentication Problems

When technical setup fails.

Diagnosing Authentication Issues

Check SPF:

Tools: MXToolbox, Google Admin Toolbox
What to look for:
- SPF record exists
- All sending IPs included
- Not exceeding 10 DNS lookups
- Pass status in headers

SPF Problems and Fixes:

Check DKIM:

Tools: MXToolbox, mail-tester.com
What to look for:
- DKIM signature present
- Signature validates
- Key published in DNS
- Alignment with From domain

DKIM Problems and Fixes:

Check DMARC:

Tools: MXToolbox, DMARC Analyzer
What to look for:
- Record published
- Policy setting
- Reporting configured
- Alignment passing

DMARC Problems and Fixes:

Authentication Testing Process

Step-by-Step:

1. Send test email to mail-tester.com
2. Review authentication results
3. Check raw headers in received email
4. Use MXToolbox to verify DNS records
5. Fix identified issues
6. Retest

Reputation Problems

When ISPs don't trust you.

Diagnosing Reputation Issues

Check Sending Reputation:

Google Postmaster Tools:

• Domain reputation (High, Medium, Low, Bad)
• IP reputation
• Spam rate
• Authentication rates

Microsoft SNDS:

• IP status
• Complaint rate
• Trap hits
• Filter results

Third-Party Tools:

• Sender Score (Validity)
• Talos Intelligence (Cisco)
• BarracudaCentral

Reputation Indicators:

Common Reputation Issues

Issue: Sudden Reputation Drop

Possible Causes:

• Spam trap hit
• Complaint spike
• Large bounce event
• Blocklist listing

Diagnostic Steps:

1. Check for recent blocklist additions
2. Review complaint rates in feedback loops
3. Analyze recent bounce patterns
4. Look for changes in sending patterns

Fixes:

• Pause sends to affected domain
• Clean list aggressively
• Request blocklist removal
• Gradually rebuild sending

Issue: Gradual Reputation Decline

Possible Causes:

• Declining engagement
• List aging without cleaning
• Increasing complaints
• Sending volume changes

Diagnostic Steps:

1. Chart engagement over time
2. Analyze list age distribution
3. Review complaint trends
4. Compare volume to engagement

Fixes:

• Re-engage or remove inactive subscribers
• Implement regular list hygiene
• Reduce frequency to unengaged
• Adjust content strategy

Blocklist Issues

Identifying Blocklist Problems:

Check: MXToolbox Blocklist Check
Enter: Your sending IP or domain
Review: All major blocklists

Major Blocklists:

• Spamhaus (most important)
• Barracuda
• SORBS
• SpamCop
• UCEProtect

Blocklist Removal Process:

1. Identify the blocklist
2. Find their removal process
3. Fix the underlying issue first
4. Submit removal request
5. Monitor for re-listing

Spamhaus Removal:

• Self-service for most listings
• Requires identifying and fixing cause
• CSS/CBL require specific actions
• SBL may require direct contact

List Quality Problems

When your data is the issue.

Diagnosing List Problems

High Bounce Rates:

Types of Bounces:

• Hard bounces: Address doesn't exist
• Soft bounces: Temporary delivery issues
• Block bounces: Rejected by server

Analysis:

Group bounces by:
- Type (hard vs. soft)
- Domain
- List segment
- Acquisition source

Common Patterns:

Low Engagement:

Symptoms:

• Open rates well below industry benchmarks
• Click rates minimal
• No replies or interactions
• Conversions from email near zero

Causes:

• Unengaged subscribers
• Poor inbox placement
• Irrelevant content
• Wrong audience

Diagnostic:

1. Segment by engagement level
2. Check placement (use seed testing)
3. Survey subscribers
4. Review content relevance

List Cleaning Process

Immediate Actions:

• Remove all hard bounces
• Remove repeated soft bounces
• Remove obvious invalid formats
• Remove role addresses (info@, admin@)

Verification:

• Run entire list through verification service
• Remove invalid, risky, and unknown addresses
• Keep only verified deliverable addresses

Ongoing:

• Verify emails at point of capture
• Regular re-verification (quarterly)
• Remove long-term unengaged
• Monitor ongoing metrics

Content Problems

When content triggers filters.

Diagnosing Content Issues

Signs of Content Problems:

• Specific emails go to spam, others don't
• Same list, different results by content
• A/B tests show content-based differences
• No authentication or reputation issues

Content Analysis:

1. Run email through spam checker (mail-tester.com)
2. Check SpamAssassin score
3. Review trigger word presence
4. Analyze HTML/text ratio
5. Check links and domains

Common Content Triggers

Words and Phrases:

• "FREE!!!" (excessive caps/punctuation)
• "Act now"
• "Limited time"
• "Click here"
• "Congratulations"
• "Winner"

Note: Context matters more than individual words. Natural use is usually fine; excessive or manipulative use triggers filters.

Formatting Issues:

• Too many images, too little text
• All caps text
• Excessive punctuation (!!!)
• Large number of links
• Bright red text
• Tiny text (hidden content)

Technical Issues:

• Broken HTML
• Missing text version
• Too large file size
• Suspicious attachments
• Redirecting links

Content Fixes

Subject Lines:

• Avoid spam trigger words
• Don't use ALL CAPS
• Limit punctuation
• Keep reasonable length
• Be honest about content

Email Body:

• Balance images and text
• Use clean HTML
• Include text version
• Minimize links
• Use reputable link domains

Testing:

• Always test before sending
• Use spam checker tools
• Check multiple ISPs
• A/B test variations

Technical Problems

When infrastructure fails.

Server and Configuration Issues

Sending Server Problems:

• Server blacklisted
• Incorrect configuration
• SSL/TLS issues
• Rate limiting

Diagnostic Commands:

# Check MX records
dig MX example.com

# Check SPF
dig TXT example.com

# Check DKIM
dig TXT selector._domainkey.example.com

# Test SMTP connection
telnet mail.example.com 25

ESP Issues:

• Platform deliverability problems
• Shared IP reputation
• Configuration errors
• Account issues

DNS Problems

Common DNS Issues:

• Records not propagated
• Incorrect syntax
• Missing records
• Conflicting records

DNS Troubleshooting:

1. Check DNS propagation (whatsmydns.net)
2. Verify record syntax
3. Test with multiple DNS servers
4. Compare expected vs. actual values

Rate Limiting and Throttling

Symptoms:

• Emails delivered slowly
• Some bounces with "too many connections"
• Inconsistent delivery times

Causes:

• Sending too fast
• Sending too much volume
• New IP/domain not warmed
• ISP-specific limits

Fixes:

• Reduce sending rate
• Spread sends over time
• Warm up new IPs/domains properly
• Respect ISP guidelines

ISP-Specific Problems

When issues affect certain mailbox providers.

Gmail Issues

Diagnostic:

• Google Postmaster Tools required
• Check domain and IP reputation
• Review spam rate

Common Gmail Problems:

Gmail Best Practices:

• Use Google Postmaster Tools
• Maintain engagement
• Authenticate properly
• Follow Sender Guidelines
• Enable one-click unsubscribe

Microsoft (Outlook.com, Hotmail) Issues

Diagnostic:

• Microsoft SNDS
• Junk Mail Reporting Program (JMRP)

Common Microsoft Problems:

Microsoft Best Practices:

• Register with SNDS
• Join JMRP feedback loop
• Follow Microsoft guidelines
• Contact postmaster for issues

Yahoo Issues

Diagnostic:

• Yahoo Postmaster
• Feedback loop data

Common Yahoo Problems:

• Aggressive filtering
• Reputation sensitivity
• Rate limiting

Yahoo Best Practices:

• Register with Yahoo Postmaster
• Implement feedback loop
• Maintain low complaint rates
• Follow Yahoo guidelines

Troubleshooting Workflow

Systematic approach to solving problems.

Step-by-Step Process

Step 1: Identify the Problem

- What metric is affected?
- When did it start?
- How severe is it?
- Who/what is affected?

Step 2: Gather Evidence

- Pull relevant metrics
- Check authentication tools
- Review reputation dashboards
- Analyze bounce messages
- Test current emails

Step 3: Form Hypothesis

Based on evidence, the most likely cause is:
[ ] Authentication issue
[ ] Reputation problem
[ ] List quality issue
[ ] Content trigger
[ ] Technical failure

Step 4: Test Hypothesis

- Isolate the variable
- Run controlled test
- Compare results
- Confirm or revise hypothesis

Step 5: Implement Fix

- Apply targeted solution
- Monitor for improvement
- Document changes
- Set up prevention

Step 6: Verify Resolution

- Metrics returning to baseline?
- Issue no longer occurring?
- No new problems created?
- Prevention in place?

Decision Tree

Low Open Rates?
├── Check authentication → Failing? → Fix auth
├── Check reputation → Low? → Improve reputation
├── Check deliverability → Landing in spam? → Address spam triggers
└── Check content → Poor subject lines? → Improve subject lines

High Bounces?
├── Mostly hard bounces? → Verify and clean list
├── Specific domains? → Check blocklists
├── Recent list change? → Review new data source
└── Sudden increase? → Check for technical issues

High Complaints?
├── Content complaints? → Review content value
├── Frequency complaints? → Reduce frequency
├── Unknown sender? → Improve from name recognition
└── Permission issues? → Review consent practices

Prevention Strategies

Stopping problems before they start.

Monitoring Systems

Set Up Alerts For:

• Open rate drops >20%
• Bounce rate exceeds 2%
• Complaint rate exceeds 0.1%
• Blocklist additions
• Authentication failures

Regular Checks:

• Daily: Delivery metrics
• Weekly: Reputation dashboards
• Monthly: Full deliverability audit
• Quarterly: List verification

Best Practices

Authentication:

• Implement SPF, DKIM, DMARC
• Monitor authentication reports
• Update records when adding senders
• Progress DMARC to p=reject

Reputation:

• Maintain low complaint rates
• Keep bounce rates minimal
• Monitor and respond to feedback
• Warm up new IPs/domains

List Quality:

• Verify at capture
• Clean regularly
• Remove unengaged
• Maintain consent records

Content:

• Test before sending
• Use spam checkers
• Follow best practices
• Monitor performance

Troubleshooting Checklist

Initial Assessment

• [ ] Identified specific metrics affected
• [ ] Determined when problem started
• [ ] Identified scope (all emails or specific)
• [ ] Gathered baseline comparisons

Authentication Check

• [ ] SPF record valid and passing
• [ ] DKIM configured and passing
• [ ] DMARC published and aligned
• [ ] No authentication errors in logs

Reputation Check

• [ ] Google Postmaster reviewed
• [ ] Microsoft SNDS reviewed
• [ ] Blocklist status checked
• [ ] Sender Score checked
• [ ] Complaint rates acceptable

List Quality Check

• [ ] Bounce rate under 2%
• [ ] Hard bounces removed
• [ ] List recently verified
• [ ] Engagement rates healthy

Content Check

• [ ] Spam score acceptable
• [ ] No obvious triggers
• [ ] HTML clean and valid
• [ ] Links working and reputable

Technical Check

• [ ] Server properly configured
• [ ] DNS records correct
• [ ] No infrastructure issues
• [ ] ESP status verified

Data Quality as Prevention

Why verification prevents problems.

The Root Cause Issue

Many Deliverability Problems Start With:

• Invalid addresses on list
• Spam traps from old data
• High bounce rates
• Degraded engagement metrics

Verification Prevents:

• Bounce-related reputation damage
• Spam trap hits
• Wasted sends
• Skewed metrics

Verification Strategy

At Capture: Verify every email before it enters your list using our email checker.

Regular Cleaning: Re-verify entire list quarterly with bulk verification.

Before Major Campaigns: Verify list before high-stakes sends.

Post-Issue: After any deliverability problem, verify and clean immediately.

Conclusion

Deliverability troubleshooting requires systematic diagnosis before applying fixes. Understanding the category of problem—authentication, reputation, list quality, content, or technical—guides you to the right solution.

Key troubleshooting principles:

1. Gather data first: Know what's actually happening before guessing
2. Categorize the problem: Different issues require different fixes
3. Test hypotheses: Verify your diagnosis before major changes
4. Fix systematically: Address root causes, not just symptoms
5. Prevent recurrence: Build systems to catch problems early

Most deliverability problems trace back to list quality. Invalid addresses cause bounces, hurt reputation, and trigger defensive filters. Following email list hygiene best practices prevents most deliverability issues.

Implement real-time email verification to stop problems before they start, and use bulk verification for regular list cleaning. Combine troubleshooting with our email deliverability guide and bounce rate reduction strategies. Ready to prevent deliverability problems at the source? Start with BillionVerify to verify your email list and maintain inbox placement.