Unable to receive mail on example.com

Symptoms

• Mail for example.com is not received without any records in /var/log/maillog.

• The following error can be found on the sender's side:
  

  CONFIG_TEXT: connect to example.com[203.0.113.3]:25: Network is unreachable

  Where 203.0.113.3 is the IP of ServerShield by Cloudflare extension.
• Port 25 is open on the server:

  # nmap -p 25 203.0.113.2 -Pn
  PORT STATE SERVICE
  25/tcp open smtp

• Domain example.com does not have MX record:

  # dig MX example.com +short
  #

Cause

Missing MX record for example.com

Resolution

If Plesk DNS server is used, add MX record for the domain using the article: How to add a DNS record for a domain in Plesk?

In case an external DNS server is used, it is required to add the appropriate MX records on the registrar's side using the article: How to configure mail in Plesk if DNS is hosted externally?

Note: It is required to wait for the DNS propagation period (up to 48 hours, but mostly faster).