Email - Submission Port (587,25)

About

submission port are the port of the mail server where an email client would connect to try to send an email.

Port

There is basically two well-known ports.

25

The port 25 is a submission port for a basic connection (ie without TLS) without authentication.

The localhost port 25 is used by Postfix or similar mail server running on the local machine.

  • For remote client, it denies relay and accepts only to receive email for the domain that it manages.
  • For local client, it accepts to send email.

587

The port 587

  • is considered a submission port for SSL (TLS). It is also what remote clients use to send mail out using your server.
  • should only allow authenticated connections

Configuration

Postfix

To open the port 587, with postfix, the submission line should be uncommented and parameters should be overwritten.

# ==========================================================================
# service type  private unpriv  chroot  wakeup  maxproc command + args
#               (yes)   (yes)   (yes)   (never) (100)
# ==========================================================================
submission inet n      -       n       -       -       smtpd
  -o smtpd_tls_security_level=encrypt
  -o syslog_name=postfix/submission
  -o smtpd_sasl_auth_enable=yes
  -o smtpd_reject_unlisted_recipient=no
  -o smtpd_client_restrictions=$mua_client_restrictions
  -o smtpd_helo_restrictions=$mua_helo_restrictions
  -o smtpd_sender_restrictions=$mua_sender_restrictions
  -o smtpd_recipient_restrictions=permit_sasl_authenticated,reject
  -o milter_macro_daemon_name=ORIGINATING

Specification

If you want to known more about the subject, there is also a whole specification. See RFC6409 - Message Submission for Mail


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