Mail service is not available on Plesk server: non-null host address bits in IP address

Symptoms

• Unable to send emails from a server running Postfix.
• The following errors appear in
/var/log/maillog
:

  mail postfix/smtpd[19055]: fatal: non-null host address bits in "203.0.113.2/24", perhaps you should use "203.0.113.0/24" instead
   mail postfix/smtpd[19055]: NOQUEUE: reject: RCPT from server[203.0.113.4]: 451 4.3.0 <server[203.0.113.4]>: Temporary lookup failure; from=<[email protected]> to=<[email protected]> proto=ESMTP helo=<mail.example.com>

Cause

Incorrect configuration of Postfix
main.cf
file.

Resolution

1. Connect to the server via SSH;

   Note: if direct SSH access to the server is not possible, contact server administrator for further assistance.

2. Check Postfix configuration file:

   # grep -w mynetworks /etc/postfix/main.cf | grep -v “#”
   mynetworks = 127.0.0.0/8, [::1]/128, 203.0.113.2/24

3. Change the
203.0.113.2./24
value to
203.0.113.2/32
or
203.0.113.0/24, if it is required to allow send emails from whole subnet segment.

4. Restart postfix:

   # service postfix restart