Webtop email filtering

I’ve started to experiment with email filtering in webtop, but it doesn’t seem to work.

I have a user mail box that receives catch all emails, the mailbox is called mail@domain.com and email that doesn’t go to a user goes to this mail box.

Webtop is configured is the email client for the mail@domain.com mailbox and i’ve configured some filters that should route email to specific address into specific folders, e.g. in the mail@domain.com catch all mail box I have a rule that should route email to school@domain.com into a folder Inbox\school. But the filter doesn’t work.

When an email arrives that should match the filter rule, i see this in /var/log/maillog
Jun 4 10:00:04 nethserver dovecot: lmtp(school@domain.com): +EimIhS42F4JawAAOPg6aQ: sieve: Execution of script /var/lib/nethserver/sieve-scripts/unknown.sieve failed, but implicit keep was successful (user logfile /var/lib/nethserver/sieve-scripts/unknown.sieve.log may reveal additional details)

/var/lib/nethserver/sieve-scripts/unknown.sieve says the following (i’ve changed the actual message ID)

sieve: info: started log at Jun 04 10:00:07.
error: msgid=<PR1PR06MB5579ACB57453EA11A26F7CA7C6890@sendingdomain.com>: failed to store into mailbox 'mail@domain.com': Mailbox doesn't exist: mail@domain.com.

I’ve created a sieve rule for a normal user, and that filter works.
Is this problem something to do with the way a catchall mail box is configured?

what’s going on?
what should i do to learn why sieve filtering doesn’t work?

thanks and regards