If for some reasons you’re having problems with the mail issues related to qmail server, you can repair and rebuild the qmail configuration files on your linux plesk dedicated server and at the same time restore settings for all the mailboxes which are present on your server.

To get this done, plesk have an utility named mchk and you can use it as below:

Restore all settings except for SpamAssassin configuration

root# /usr/local/psa/admin/bin/mchk --without-spam

Restore all settings

root# /usr/local/psa/admin/bin/mchk --with-spam

Restore only SpamAssassin settings

root# /usr/local/psa/admin/bin/mchk --spam-only