bug in mailserver settings

setup:
plesk 17.0.17 mu12
centos6.latest
postfix
courier imap
127.0.0.0 / 8 and ::1 / 128 in whitelist
webmail stock out of the box, no changes made to the config.

replication:
Tools & Settings
Serverwide Mail Settings
click OK, no changes needed

error:
from now on webmail gives an authentication error when sending to an external email address.

fix:
at commandline run:

plesk repair mail
or
/usr/local/psa/admin/sbin/mchk --with-spam

result:
webmail can send mail again to external email...

bug in mailserver settings