Network ports · 25
25 — SMTP
Port 25 is the classic SMTP port for server-to-server mail delivery.
Key facts
| Port | 25 |
|---|---|
| Transport | TCP |
| Service | SMTP relay |
| Authentication | Often none between servers |
| Encryption | STARTTLS when offered |
| Range | Well-known (0–1023) |
Server-to-server delivery
When one mail server delivers a message to another, it connects to port 25 and speaks SMTP. Authentication is not part of the base protocol; trust comes from the sending server's IP reputation, SPF, DKIM and DMARC, not from a password.
Many residential and cloud providers block outbound port 25 to limit spam. That is why application mail should go through a submission service rather than a direct connection to the recipient's MX.
Submission is a different port
Mail clients submit outgoing messages on port 587, where authentication is required and STARTTLS is expected. Port 465 is implicit TLS submission. Using port 25 for a client is a common misconfiguration that either fails or gets the client treated as a spam source.
Whatever the port, run the receiving side with TLS, authenticate where the protocol allows it, and publish SPF, DKIM and DMARC so receivers can evaluate your mail.
Frequently asked questions
Why can I not send mail from my laptop on port 25?
Many access providers block it to reduce spam. Use authenticated submission on 587 or 465.
Does port 25 require authentication?
Not in the base protocol between servers. Client submission on 587 does require it.