communication email_config ai_generated true

SMTP connection fails — TLS vs STARTTLS port confusion (465 vs 587)

ID: communication/smtp-tls-starttls-port-confusion

Also available as: JSON · Markdown

90%Fix Rate

92%Confidence

4Evidence

2023-01-01First Seen

Version Compatibility

Version	Status	Introduced	Deprecated	Notes
any	active	—	—	—

Using implicit TLS (port 465) settings with STARTTLS port (587) or vice versa.

generic

93% success Use port 587 with STARTTLS for submission (RFC 8314)
```
smtplib.SMTP('smtp.example.com', 587); server.starttls()
```
Sources: https://www.rfc-editor.org/rfc/rfc8314
88% success Use port 465 with implicit TLS when STARTTLS is not supported
```
smtplib.SMTP_SSL('smtp.example.com', 465)
```
Sources: https://www.rfc-editor.org/rfc/rfc8314

Common approaches that don't work:

Use port 465 with STARTTLS 95% fail
Port 465 expects implicit TLS; STARTTLS is an upgrade from plaintext
Use port 587 with implicit SSL/TLS 95% fail
Port 587 starts plaintext and upgrades via STARTTLS