Incoming/outgoing emails cannot be sent/delivered, but mail service is configured properly

Symptoms

• Incoming/outgoing emails cannot be sent/delivered, but mail service is configured properly;

• It is not possible to log into a desktop or mobile mail client, for example below error can occurs:

  Couldn't verify account  

• It is not possible to send/receive mails;

• Mail services are up and running and required ports are listened to:

  # netstat -nltp | egrep '25|110|143|465|993|995'
  tcp 0 0 0.0.0.0:25 0.0.0.0:* LISTEN 9007/master
  tcp 0 0 0.0.0.0:993 0.0.0.0:* LISTEN 1557/dovecot
  tcp 0 0 0.0.0.0:995 0.0.0.0:* LISTEN 1557/dovecot
  tcp 0 0 0.0.0.0:110 0.0.0.0:* LISTEN 1557/dovecot
  tcp 0 0 0.0.0.0:143 0.0.0.0:* LISTEN 1557/dovecot
  tcp 0 0 0.0.0.0:465 0.0.0.0:* LISTEN 9007/master

• These ports are not restricted by firewall locally, for example:

  # iptables -L -n | egrep '25|110|143|465|993|995'
  ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:25
  ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:465
  ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:110
  ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:995
  ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:143
  ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:993

• These ports are not available externally:

  # nmap -p25,110,143,465,993,995 203.0.113.2
  PORT STATE SERVICE
  25/tcp filtered smtp
  110/tcp filtered pop3
  143/tcp filtered imap
  465/tcp filtered smtps
  587/tcp filtered submission
  993/tcp filtered imaps
  995/tcp filtered pop3s

• Cannot connect Outlook to mail account:

  error (0x8004210A) : 'The operation timed out waiting for a response from the receiving (POP) server.

Cause

Ports required for mail services are limited with intermediate firewall.

Resolution

Check settings on the intermediate firewall or contact the Hosting Provider to open the following ports:

25
110
143
465
993
995

Additional info

Which Ports Need To Be Opened for all Plesk Services to Work with a Firewall?