Email Delivery Troubleshooting Guide
Email delivery issues can range from simple configuration problems to complex reputation issues. This guide provides systematic troubleshooting workflows to diagnose and fix the most common email delivery problems.
Troubleshooting Decision Tree
Common SMTP Error Codes
5.7.x Security and Policy Errors
| Code | Description | Common Cause | Solution |
|---|---|---|---|
| 550 5.7.1 | Message rejected | SPF/DMARC failure, blacklisted | Check authentication, blacklists |
| 550 5.7.23 | SPF validation failed | Sending IP not in SPF | Add IP to SPF record |
| 550 5.7.26 | DMARC failure | Auth failed or not aligned | Fix SPF/DKIM alignment |
| 553 5.7.1 | Sender address rejected | Invalid From address | Use valid sender domain |
| 554 5.7.1 | Relay access denied | Not authorized to send | Configure proper relay auth |
5.1.x Address Errors
| Code | Description | Common Cause | Solution |
|---|---|---|---|
| 550 5.1.1 | User unknown | Email address doesn't exist | Verify recipient address |
| 550 5.1.2 | Domain not found | Invalid domain | Check domain spelling |
| 550 5.1.8 | Sender address rejected | Invalid sender | Fix From address |
5.2.x Mailbox Errors
| Code | Description | Common Cause | Solution |
|---|---|---|---|
| 550 5.2.1 | Mailbox disabled | Account suspended | Contact recipient |
| 552 5.2.2 | Mailbox full | Over quota | Recipient must clear space |
| 552 5.2.3 | Message too large | Exceeds size limit | Reduce attachment size |
4.x.x Temporary Errors
| Code | Description | Common Cause | Solution |
|---|---|---|---|
| 421 | Service unavailable | Server busy | Wait for auto-retry |
| 450 | Mailbox unavailable | Temporary issue | Wait for auto-retry |
| 451 | Local error | Processing issue | Check later |
| 452 | Insufficient storage | Disk space | Wait for cleanup |
Authentication Troubleshooting
SPF Failures
DKIM Failures
"v=DKIM1; p="
DMARC Failures
Blacklist Troubleshooting
Checking Blacklist Status
Common Blacklists and Delisting
| Blacklist | Check URL | Delisting Process |
|---|---|---|
| Spamhaus | spamhaus.org/lookup | Submit removal request, fix issue |
| Barracuda | barracudacentral.org/lookups | Register, request removal |
| SpamCop | spamcop.net/bl.shtml | Usually auto-expires in 24h |
| SORBS | sorbs.net | Request delisting via web form |
| Proofpoint | ipcheck.proofpoint.com | Contact Proofpoint support |
| Microsoft | sender.office.com | Submit through portal |
Delisting Steps
Spam Folder Issues
Why Emails Land in Spam
Gmail Deliverability
Microsoft 365 Deliverability
Diagnostic Commands
DNS Checks
0 . "v=spf1 -all" "v=DKIM1; p=" "v=DMARC1;p=reject;sp=reject;adkim=s;aspf=s"
SMTP Testing
Email Header Analysis
Prevention Best Practices
Email Infrastructure Checklist
Monitoring Setup
Related Resources
- Email Authentication Complete Guide - Hub article
- SPF Record Syntax Guide - SPF details
- SPF Propagation Time - DNS timing
- Email Authentication Headers - Header analysis
- Email Deliverability Guide - Improve rates
Tools
- DNS Lookup - Check DNS records
- MX Record Checker - Verify mail routing
- SPF Record Generator - Create SPF records
- DMARC Record Generator - Build DMARC policies