I am now using dovecot and its working very well :)
but now there's another problem. The isp's email-username is limited to n-characters so I get in trouble while using 'large-email-adresses'
in another post, grant is telling about a solution how to fix this by patching the couriers-user-workflow mentioned by desmondk in first post here.
perhaps you know a working solution for dovecot? Would like to use it furthermore cause there's a webmin module available :)