๐ ~1 min read
Table of contents
Symptom & Impact
Outgoing email remains deferred for long periods and transactional notifications are delayed.
Environment & Reproduction
Appears on newly deployed mail relays with incomplete DNS records or blocked SMTP egress.
Root Cause Analysis
Destination lookup or SMTP connectivity fails, causing Postfix to keep messages in deferred queue.
Quick Triage
Run postqueue -p and tail /var/log/mail.log to identify repeated delivery failure reasons.
Step-by-Step Diagnosis
Test DNS resolution with dig and host commands, and verify outbound TCP 25 or relay endpoint access.
Solution – Primary Fix
Correct DNS and relayhost settings in /etc/postfix/main.cf, run sudo postfix reload, then flush queue with sudo postqueue -f.
Still having issues? Our IT Solutions & Services team can diagnose and resolve this for you. Get in touch for a free consultation.
Solution – Alternative Approaches
Route mail through authenticated smarthost on submission port when direct SMTP is filtered.
Verification & Acceptance Criteria
Deferred queue drains and new messages deliver successfully with clean mail.log status entries.
Rollback Plan
Restore prior main.cf and master.cf backups and reload Postfix if new relay settings fail.
Prevention & Hardening
Monitor queue depth and DNS health, and configure alerting on sustained defer counts.
Related Errors & Cross-Refs
Common lines include ‘Name service error’ and ‘Connection timed out’ during SMTP delivery.
Related tutorial: View the step-by-step tutorial for Ubuntu 26.04 LTS.
View all Ubuntu 26.04 LTS tutorials on the Tutorials Hub โ
Browse all common problems & solutions on the Tutorials Hub.
References & Further Reading
Postfix BASIC_CONFIGURATION_README, Ubuntu mail server documentation, and DNS troubleshooting guides.
Need Expert Help?
If you cannot resolve this yourself, our team offers hands-on Server Management, Managed IT Services, and flexible Support Plans. Contact us today โ we respond within one business day.
