SMTP hostname and port numbers

The SMTP API is accessible through the "" domain name. There are different ports to use for messaging: port 25, 587 and 2525 are all used with STARTTLS encryption and are handled in the same manner. It is also possible to use port 465 without STARTTLS, see the section on port 465 for more information.

Port:       25, 587, 2525
Encryption: STARTTLS

Only authenticated and encrypted connections using "STARTTLS" are supported on ports 25, 587 and 2525. Non-authenticed emails and messages that are not sent over a secure connection are rejected.

Note that we can reject mail during the "RCPT TO" phase of the SMTP protocol, or until after you've sent all message data. Sometimes we process the entire mail message to find out who is trying to abuse our systems and reject the mail only after we've seen the full message.

What is the difference between port 25, 587, and 2525?

You can use port 25, 587, and 2525 to send mail to SMTPeter. There is no difference between these ports. We have opened ports 587 and 2525 because some providers block access to port 25. Port 2525 is especially useful for Google Cloud users since Google Cloud does neither support 25 nor 587.

Port 465

As an alternative to the ports mentioned previously, you can also connect to port 465. This opens up a TCP connection in a secure state right away, and skips the STARTTLS handshake. Although sending mail over port 465 was never standardized and is even deprecated in favor of the STARTTLS encryption (in fact, port 465 has even been reassigned to a new service), communication over port 465 is faster because the STARTTLS handshake is skipped. It is also more secure because it is impossible to intercept the "EHLO/HELO" handshake.

Port:       465
Encryption: SMTPS

More information