Emails sent to a domain that is hosted in Plesk are bounced back: mail for example.com loops back to myself

Symptoms

• A domain example.com uses the remote mail server;

• Mail service is disabled in Domains > example.com > Mail Settings;

• When email messages are sent to this domain from mailbox located on the same server, messages are bounced with the following error in /var/log/maillog:

  postfix/qmgr[20813]: 408755E04C2: from=<jdoe@example.org>, size=635, nrcpt=1 (queue active)
  postfix/smtp[19079]: 408755E04C2: to=<jdoe@example.com>, relay=none, delay=0.14, delays=0.14/0/0/0, dsn=5.4.6, status=bounced (mail for example.com loops back to myself)

• MX record of the domain example.com cannot be resolved by default DNS server and its A record points to Plesk server:

  # host -t MX example.com
  example.com has no MX record
  # host example.com
  example.com has address 192.0.2.2
  # hostname -I
  192.0.2.2

• Correct MX record is returned if a different DNS server is specified:

  # host -t MX example.com 8.8.8.8
  Using domain server:
  Name: 8.8.8.8
  Address: 8.8.8.8#53
  Aliases:

  example.com mail is handled by 10 externalmail.example.com.

Cause

Default DNS server returns incorrect answers if the affected domain is queried, as a result, Postfix tries to connect to itself.

Resolution

1. Connect to the server via SSH

2. Change the DNS server (nameserver) in /etc/resolv.conf file to a valid DNS server, such as 1.1.1.1:

   # cat /etc/resolv.conf
   nameserver 1.1.1.1

   Note: Remove localhost or 127.0.0.1 if it is configured as a nameserver in /etc/resolv.conf file.

   Note: If this file is managed by an external program, such as NetworkManager, or resolvconf, configuration of the respective program should be modified.

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