Unable to receive or send e-mails: port 25 is blocked

Symptoms

• Unable to receive/send e-mails.

• Emails cannot be sent by using PHP script.

• It is not possible to connect to the server by telnet on port 25:

  # telnet 192.0.2.2 25
  Trying 192.0.2.2...
  telnet: connect to address 192.0.2.2: Connection refused

• Port 25 appears filtered when trying to connect from outside:

  # nmap -p25,465,587 192.0.2.2
  ...
  PORT STATE SERVICE
  25/tcp filtered smtp
  465/tcp filtered smtps
  587/tcp filtered submission

  Nmap done: 1 IP address (1 host up) scanned in 0.59 seconds

• Outgoing mail connections via 25 port are not possible:

  # telnet mail.example2.com 25
  Trying 203.0.113.2...
  telnet: connect to address 203.0.113.2: Connection timed out
  Trying 2001:db8:f61:a1ff:0:0:0:80... telnet: connect to address 2001:db8:f61:a1ff:0:0:0:80 Network is unreachable.

• It is possible to send out an email message locally using the command:

  # telnet localhost 25

Cause

SMTP ports is blocked by a firewall.

Resolution

• If the Plesk Firewall is used, open mail port under Plesk > Tools & Settings > Firewall;
• If the system or a third party firewall is active on the server, make sure that it is not blocking the connection to the 25 port.
• In case of an intermediate firewall, it is required to contact the service provider.
• For AWS instance, open port 25 according to Amazon documentation.
• For Azure instance, open port 25 according to Azure documentation. 

Additional information

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