Port 25 is the default SMTP port used for communication between sending and receiving email servers, but it's not directly used for SMTP authentication.
Here's a breakdown:
Port 25's Role:
- Mail Delivery: Port 25's primary function is to facilitate the transfer of email messages from one mail server to another. According to the provided reference, it is used to "enable communication between the sending and receiving servers when delivering an email message to a recipient."
- Server-to-Server: This port is used to relay email messages from one server to another and is not used for authentication.
- Blocking: Many ISPs block incoming connections on port 25 as a security measure. This is because spammers often exploit port 25 to send unsolicited emails.
SMTP Authentication Ports:
- Port 587 (Submission Port): This is the recommended port for SMTP authentication and email submission by mail clients. It is often paired with TLS/SSL encryption.
- Port 465 (Deprecated, but sometimes used): This port was originally intended for SMTPS (SMTP over SSL), but it has been officially deprecated. Some older systems might still use it.
SMTP: A table of ports
| Port | Description |
|---|---|
| 25 | SMTP (Simple Mail Transfer Protocol): Used for server-to-server email relaying. |
| 587 | Message Submission: Authenticated email submission by a client to a mail server, commonly used with TLS/SSL. |
| 465 | SMTPS (SMTP over SSL): Deprecated, but some older systems might still use it. |
In short, port 25 is for mail relay, not authentication. SMTP authentication typically occurs on ports 587 (submission) or, in some legacy cases, 465.
