Hi
Can somone with Plesk take a look in /var/qmail/alias

cd /var/qmail/alias
cat .qmail-mailer-daemon
cat .qmail-mailer-postmaster
cat .qmail-mailer-root


and please see if you have a "&" before the
email address like examples below

&root@domain.net
&postmaster@domain.net
&mailer-daemon@domain.net

or you have just the email address postmaster@domain.net

qmail allows the use of both syntaxes.

Redirects can be done with or without &, both ways are valid.