sieve for systemusers

Hello!

I enabled LDA for systemusers bei adding mailbox_command = /usr/lib/dovecot/dovecot-lda -f "$SENDER" -a "$RECIPIENT" to main.cf of postfix.

I have done this because I want to use sieve for systemusers too, not only for virtual users.

But the sievescripts do not work for those systemusers.

What else needs to be done to get sieve to work for systemusers?

Kind regards

Andreas