How to Troubleshoot Email Bouncing and Rejection on Linux

Email bouncing is one of the most common mail server problems. Understanding bounce codes and systematically checking each potential cause will resolve most issues.

Understanding Bounce Codes

# Common SMTP error codes
421 - Service temporarily unavailable (try later)
450 - Mailbox unavailable (temporary)
451 - Local error (retry)
452 - Insufficient storage
550 - Mailbox unavailable (permanent)
551 - User not local
552 - Mailbox full
553 - Mailbox name invalid
554 - Transaction failed (often blacklist)

Step 1 — Check Mail Logs

# Postfix
sudo tail -100 /var/log/mail.log | grep -E "bounce|reject|defer"

# Exim (cPanel)
sudo tail -100 /var/log/exim_mainlog | grep -E "bounce|reject|defer"

Step 2 — Check if IP is Blacklisted

# Check multiple blacklists at once
# Visit: https://mxtoolbox.com/blacklists.aspx

# Or check specific RBLs
dig +short YOUR_SERVER_IP.zen.spamhaus.org
dig +short YOUR_SERVER_IP.bl.spamcop.net

# If response is 127.x.x.x — you are listed

Step 3 — Fix Blacklisting

# Request removal from Spamhaus
https://www.spamhaus.org/query/ip/YOUR_IP

# Request removal from SpamCop
https://www.spamcop.net/bl.shtml

Step 4 — Verify DNS Records

# Check PTR (reverse DNS)
dig -x YOUR_SERVER_IP

# Check MX records
dig MX yourdomain.com

# Check SPF
dig TXT yourdomain.com | grep spf

# Test all at once
https://mxtoolbox.com/SuperTool.aspx

Step 5 — Check Mail Queue

# View queued messages
postqueue -p

# Force retry all deferred
postqueue -f

# View specific message
postcat -vq MESSAGE_ID

Step 6 — Test Email Score

# Send email to mail-tester.com unique address
# Visit: https://www.mail-tester.com
# Aim for score 10/10

Conclusion

Email deliverability issues require systematic diagnosis. Our team provides emergency mail server support with 15-minute response time.