Postfix Submission vs. Submissions (SMTPS) port

In Postfix configuration, particularly on Debian and Ubuntu systems, you might come across terms like "submission" and "submissions" when looking into the Postfix /etc/postfix/master.cf file. This article explains the differences and use cases for both ports.

Postfix Submission Port 587

• This is the standard port for email submission as defined by the Internet Message Access Protocol (IMAP).
• In Postfix, "submission" refers to the service in the master.cf file that is configured to listen on port 587.
• This port is used by email clients to send emails to a mail server. It's intended for authenticated and secure submission of email from email clients.
• Typically, it requires encryption (STARTTLS) and authentication, making it a secure way for clients to send email.

Postfix Submissions Port 465 (SMTPS)

• "Submissions" often refers to the use of port 465 for encrypted email submission.
• While port 465 was originally registered for SMTPS (SMTP over SSL), it was deprecated and then reclaimed for a new purpose, which can lead to some confusion.
• In some configurations and older setups, "submissions" may still refer to port 465, but this is less common and not standard practice.
• When used, it typically expects connections to be encrypted from the start of the connection (SMTP over SSL), in contrast to STARTTLS used on port 587.

Key Differences

• Port Number: Submission typically uses port 587, while submissions may refer to port 465.
• Encryption Method: Submission (587) often starts with an unencrypted connection upgraded to encrypted using STARTTLS, whereas submissions (465) expects an SSL/TLS encrypted connection from the start.
• Usage Standards: Port 587 is the recommended standard for client email submission, while port 465's use varies and is less common.

Where are Submission and Submissions port configured?

The configuration for the ports that Postfix listens on is configured in the master.cf file. You can view and edit that file e.g. with nano editor. The command is:

nano /etc/postfix/master.cf

In Summary

• On Debian and Ubuntu systems, when configuring Postfix, "submission" (port 587) is the standard, secure, and recommended way for email clients to submit emails to the server.
• "Submissions" might refer to configurations using port 465 but is less standard and may not be supported in all environments.