Solution
Use mchk utility to repair/rebuild all mail configuration files and restore settings for all mailboxes created in Plesk Panel:# /usr/local/psa/admin/sbin/mchk --help
Synopsis:
mchk [OPTION]
--without-spam - restore all settings except for SpamAssassin configuration
--with-spam - restore all settings
--spam-only - restore only SpamAssassin settingsThis utility rebuilds mail server control files' mailboxes settings (including alternative servers such as Postfix in modern Plesk versions). It also sets appropriate permissions for all the related files.
Usage example:
# /usr/local/psa/admin/sbin/mchk --with-spam