To repair Plesk mailbox configuration or reset them to the default settings, you can use 'mchk' utility.
Syntax : /usr/local/psa/admin/sbin/mchk [OPTION]
OPTIONS:
--without-spam : restore all settings except for SpamAssassin configuration
--with-spam : restore all settings
--spam-only : restore only SpamAssassin settings
Example:
# /usr/local/psa/admin/sbin/mchk --without-spam