Incoming e-mails are not received on a Linux Plesk server

Symptoms

• Incoming mails for all domains on a Linux Plesk server are not delivered to the mailboxes of recipients
• There are no logs related to the incoming messages in /var/log/maillog
• Test connection attempts from a remote server to the Plesk server IPs on port 25 are failing:
  

  CONFIG_TEXT: # nmap -p 25 203.0.113.2
  .....
  PORT STATE SERVICE
  25/tcp filtered smtp

• There are no firewall rules on the server itself that are blocking blocking port 25.

Cause

An external to the server (intermediate) firewall blocks the incoming SMTP connections on port 25 for your server IPs before they even reach the server.

Resolution

Contact the server provider for further assistance on allowing incoming connections on port 25 for all IP addresses that the server uses and in all firewalls that reside on levels above the server itself.