After testing this on my jail that runs postfix+dovecot, I fiddled with dovecot-lda and started using this. It worked like a charm, so I replaced my production procmail-wrapper thingy with dovecot-lda and started using Sieve. Sieve works just fine from Roundcube and the managesieve plugin. Sadly I do not see a decent option to plug it into Mail.app, which would be awesome ofcourse :). I invited some people to test the sieve filters and I wrote a nice accompanying email to my customers.
I will write the howto on how we had set this up and such. It’s pretty easy but it will help a few of you out there.
If someone knows how I can plug this into Mail.app, let me know.