The port number often tells you which role a connection is trying to perform.
| Port | Typical use |
|---|---|
| 25 | SMTP server-to-server relay |
| 465 | SMTP submission with implicit TLS |
| 587 | SMTP message submission |
| 110 | POP3 |
| 995 | POP3 with implicit TLS |
| 143 | IMAP |
| 993 | IMAP with implicit TLS |
These are standard service assignments, not a guarantee that every provider exposes every service.
Do not use port 25 for an end-user application merely because “SMTP uses 25.” Submission and relay have different trust and policy models.