Emails are not sent from a Plesk server with the errors "25: Connection timed out" and "25: Network is unreachable"

Symptoms

• Emails from a Plesk server are not sent. The following records appear in /var/log/mailllog:

  CONFIG_TEXT: connect to aspmx.l.google.com[203.0.113.2]:25: Connection timed out
  plesk postfix/smtp[17181]: connect to aspmx.l.google.com[2001:db8:f61:a1ff:0:0:0:80]:25: Network is unreachable
  plesk postfix/smtp[17181]: connect to alt2.aspmx.l.google.com[203.0.113.2]:25: Connection timed out
  plesk postfix/smtp[17181]: connect to alt2.aspmx.l.google.com[2001:db8:f61:a1ff:0:0:0:80]:25: Network is unreachable

• It is possible to connect to a remote host on port 25 using the telnet utility:

  # telnet alt2.aspmx.l.google.com 25
  ...
  Trying 74.125.28.26...
  Connected to alt2.aspmx.l.google.com.
  Escape character is '^]'.
  220 mx.google.com ESMTP c68si1753439pfa.129 - gsmtp

Cause

A private IP address is used for outgoing emails.

Resolution

1. Log in to Plesk.

2. Go to Tools and Settings > Mail Server Settings.

3. Set Outgoing mail mode to some public IP address.