How Do I Diagnose Email Delivery Problems?

Systematic Troubleshooting Approach

Step 1: Define the Problem

Clarify exactly what is happening:

• Bouncing: Emails rejected entirely
• Spam folder: Delivered but filtered
• Missing: Not arriving at all
• Delayed: Arriving hours or days late
• Provider-specific: Only affecting one mailbox provider

Step 2: Determine Scope

• All recipients or just some?
• All email or specific campaigns?
• One provider or multiple?
• When did it start?
• What changed recently?

Check Authentication First

SPF Verification

Use MXToolbox or similar tool:

• Does your SPF record exist?
• Is syntax valid?
• Are all sending sources included?
• Under 10 DNS lookups?

DKIM Verification

• Is DKIM selector published in DNS?
• Are emails being signed?
• Do signatures validate?

DMARC Verification

• Is DMARC record present?
• What is your policy (none/quarantine/reject)?
• Are SPF and DKIM aligning with the From domain?

Quick Auth Test

1. Send email to your own Gmail account
2. View "Show original" in Gmail
3. Check Authentication-Results header
4. All should show PASS

Review Reputation

Google Postmaster Tools

For Gmail delivery issues:

• Check domain reputation (High/Medium/Low/Bad)
• Check IP reputation
• Review spam rate (should be under 0.1%)
• Look for authentication failures
• Check for delivery errors

Microsoft SNDS

For Outlook/Microsoft issues:

• Review spam complaint data
• Check trap hit rates
• Monitor IP status (green/yellow/red)

Yahoo Complaint Feedback Loop

Register for Yahoo CFL to receive spam complaint notifications.

Check Blacklists

How to Check

• Use MXToolbox blacklist scanner
• Check both IP address and domain
• Scan multiple blacklist databases

If Listed

1. Identify which blacklist
2. Find the listing reason
3. Fix the underlying issue
4. Request delisting
5. Monitor for re-listing

Important Blacklists

• Spamhaus: Most impactful
• Barracuda: Widely used
• SpamCop: Complaint-based
• SORBS: Various lists

Analyze Bounce Messages

Finding Bounce Details

• Check your ESP's bounce reports
• Review bounce notification emails
• Look for SMTP error codes

Common Bounce Codes

• 550 5.1.1: User unknown (bad address)
• 550 5.7.1: Rejected for policy reasons (spam filtering)
• 421 4.7.0: Connection rate limited
• 550 5.7.25: DKIM/authentication failure
• 550 5.7.26: SPF failure

What Codes Tell You

• 4xx: Temporary issue, will retry
• 5xx: Permanent failure, will not retry
• 5.1.x: Address problems
• 5.7.x: Security/policy problems

Test Inbox Placement

Seed Testing

1. Send to test accounts at major providers
2. Check inbox vs spam placement
3. Note which providers are problematic

What to Test

• Gmail (personal and Workspace)
• Outlook.com and Microsoft 365
• Yahoo Mail
• Apple iCloud Mail

Review Recent Changes

Common Triggers

Deliverability problems often follow changes:

• New IP address: No reputation, needs warming
• New domain: No history, needs establishing
• DNS changes: May have broken authentication
• ESP migration: Configuration may be incomplete
• List import: May contain bad addresses
• Content changes: May trigger filters
• Volume increase: May exceed reputation capacity

Timeline Analysis

1. When did problems start?
2. What changed just before that?
3. Can you correlate issues with specific sends?

Content Analysis

Check for Spam Triggers

• Excessive use of spam-associated words
• All-caps subjects or text
• Excessive punctuation (!!!)
• Misleading subject lines
• Hidden text or suspicious formatting

Link Analysis

• Check all links against blacklists
• Verify link shorteners are not flagged
• Ensure redirect chains are clean

Image-to-Text Ratio

Emails that are mostly images with little text can trigger filters.

Provider-Specific Issues

Gmail Problems

• Check Google Postmaster Tools first
• Review spam rate and reputation
• Verify authentication alignment
• Check for feedback loop issues

Microsoft Problems

• Register for and check SNDS
• Consider JMRP (Junk Mail Reporting Program)
• May need to submit mitigation request

Yahoo Problems

• Register for Complaint Feedback Loop
• Check postmaster resources
• Review authentication carefully

When to Escalate

Contact Your ESP

If you use an email service provider:

• They may have additional diagnostic tools
• They can check shared IP reputation
• They may have direct relationships with mailbox providers

Contact Mailbox Providers

As a last resort:

• Use official postmaster support channels
• Provide specific examples and evidence
• Be prepared for slow responses