Email Deliverability
SMTP Timeout
An SMTP timeout occurs when the sending mail server cannot establish or maintain a connection with the receiving mail server within the expected timeframe. This results in deferred or bounced emails. Causes include network connectivity issues, firewall blocks on SMTP ports, DNS resolution failures for MX records, and overloaded mail servers.
Symptoms
- Error messages mentioning "Connection timed out" or "Connection refused"
- Emails stuck in the outbound queue with deferred status
- Intermittent delivery failures to specific domains
- Mail logs showing repeated connection attempt failures
Common Causes
- Firewall blocking outbound SMTP ports (25, 465, 587)
- Receiving mail server overloaded or temporarily down
- DNS resolution failure for the recipient's MX records
- Network path congestion or packet loss
- ISP throttling SMTP connections
Step-by-Step Troubleshooting
- 1
Check MX records
Verify the recipient domain has valid MX records pointing to active mail servers.
Use MX Record Lookup → - 2
- 3
Check port availability
Verify SMTP ports (25, 465, 587) are not blocked by firewall rules.
Use Port Scanner → - 4
Test network path
Use traceroute to identify network-level issues to the destination.
Use Ping & Traceroute → - 5
Check server health
Verify the sending server is not overloaded or running out of resources.
Recommended Diagnostic Tools
Monitor SMTP Ports
Track SMTP server availability and response times.