What are the SMTP Ports? (25, 465, 587)

25

The port 25 is a port:

• with plain connection (ie without TLS)
• without authentication.

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

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

The port 25 is mostly blocked by your Internet Service Provider (ISP) ²⁾ if you are at home. One solution is to connect to the port 587 which is a service where you need to authenticate.

Advanced tip:: In the DNS ³⁾, in a WKS record if the protocol is tcp (6), and the 26th bit is set, an SMTP server should be listening on TCP port 25

465

The port 465

• runs on a secure connection with TLS
• should allow only authenticated users
• can be used as Mail Transfer Agent (MTA) to deliver mail to another mailbox (as the users are known)
• is mostly never blocked by a firewall (ISP or others)

As your users are authenticated, you should not score their email or any traffic activities otherwise you could block your own users.

587

The port 587 has the same properties than the port section25, except that it accepts only authenticated connections.

2525

The port 2525 is a backup port for the port 25 in case the other ports are blocked by a network provider or a firewall.

2587

The port 2587 is a backup port for the port 587 in case the other ports are blocked by a network provider or a firewall.

Postfix

To open 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