Unable to send emails to Mailman mail list: Recipient address rejected: User unknown in virtual mailbox table

Symptoms

1. Mailman is installed on the server and configured in Domains > example.com > Email Addresses > Mailing Lists
2. Unable to send emails to mailing list, following error can be found in /var/log/maillog:

   hostname.example.com postfix/smtpd[9962]: NOQUEUE: reject: RCPT from mail.example.com[203.0.113.2]: 550 5.1.1 <maillist@example.com>: Recipient address rejected: User unknown in virtual mailbox table; from=<sender@gmail.com> to=<maillist@example.com proto=ESMTP helo=<some_mail_server>

3.  /var/spool/postfix/plesk/virtual does not contain mail-list address:

   # postmap -s /var/spool/postfix/plesk/virtual | grep maillist@example.com

4. Server hostname was changed recently that can be confirmed in /var/log/maillog where different hostname was displayed some time ago:

   Mar 18 05:35:29 hostname1.example.com postfix/smtpd
   Sep 08 15:22:00 hostname.example.com postfix/smtpd

Cause

Plesk bug PPPM-7494 that has been fixed in Plesk 17.8 MU 11.

Resolution

Upgrade to the latest Plesk version. 

As a workaround

1. Login into Plesk
2. Switch off/on mailing in Domains > example.com > Email Addresses > Mailing Lists > maillist@example.com