Email Server Troubleshooting Guide

Diagnose and fix the most common email delivery problems

Troubleshooting Process

Follow this systematic approach to diagnose email problems:

Step 1: Identify the symptom - What exactly fails? (connection, sending, delivery, inbox) Step 2: Check SMTP response codes - What error code does the server return? Step 3: Verify authentication - SPF, DKIM, DMARC all passing? Step 4: Check reputation - IP and domain reputation scores - Blacklist status Step 5: Test with different ISPs - Gmail inbox vs spam vs rejected? - Outlook same test?

Common Problems and Solutions

Problem: Connection Timeout

SYNTAX: Connection attempt timed out after 30 seconds

Cause: Firewall blocking port 587, SMTP server down, DNS resolution failure

Solution:

  • Verify port 587 is open in firewall
  • Test with: telnet smtp.cloudmails.eu 587
  • Check DNS: nslookup smtp.cloudmails.eu
  • Verify TLS/SSL certificates valid

Problem: 550 5.7.1 Authentication Failed

SYNTAX: "550 5.7.1 Authentication failed"

Cause: Wrong credentials, expired API key, IP not whitelisted

Solution:

  • Verify username/password matches dashboard
  • Regenerate API key if expired
  • Check if sending from whitelisted IP
  • Confirm SMTP authentication enabled

Problem: All Emails Going to Spam

SYNTAX: 100% inbox rate drops to 0%, all emails land in spam

Cause: Reputation damaged, content triggered filters, sudden volume spike

Solution:

  • Check IP reputation: postmaster.google.com
  • Reduce volume by 80% immediately
  • Review recent content for spam triggers
  • Check if on any blocklists (spamhaus.org)
  • Wait 48-72 hours for ISP to recalculate

Problem: 550 5.7.1 Message Blocked

SYNTAX: "550 5.7.1 Message blocked due to spam" or similar

Cause: Content flagged by ISP filters, suspicious links, spam words

Solution:

  • Remove excessive $ or currency symbols
  • Reduce link count (max 3-5)
  • Check for spam trigger phrases
  • Verify From/Reply-To consistency
  • Test with different content

Problem: High Bounce Rate

SYNTAX: Bounce rate exceeds ISP threshold (Gmail >0.5%)

Cause: Bad list, spam traps hit, invalid addresses, domain issues

Solution:

  • Remove hard bounces immediately
  • Run email validation on remaining list
  • Check for spam trap hits in bounces
  • Verify email addresses with SMTP check
  • Reduce sending to that ISP temporarily

Blacklist Diagnosis and Remediation

Check Major Blacklists

  • spamhaus.org - Most respected blocklist
  • sorbs.net - Often triggers false positives
  • uceprotect.net - Very aggressive, check carefully
  • mxtoolbox.com - Blacklist lookup tool

If Listed on Spamhaus

1. Check Spamhaus XBL or PBL for your IP 2. Identify the reason (spam, malware, open relay) 3. Fix the root cause 4. Submit delist request at spamhaus.org 5. Wait 24-48 hours for removal

⚠️ Never Use "Blacklist Removal Services"

These services rarely work and often scam you. The only real solution is fixing why you got listed and requesting removal through official channels.

Diagnostic Commands

# Test SMTP connection telnet smtp.cloudmails.eu 587 EHLO test.com AUTH LOGIN (enter credentials) # Check DNS/SPF nslookup -type=TXT yourdomain.com # Check DKIM nslookup -type=TXT cloudmails._domainkey.yourdomain.com # Check DMARC nslookup -type=TXT _dmarc.yourdomain.com # Test email delivery swaks --to test@gmail.com --from noreply@yourdomain.com --server smtp.cloudmails.eu

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