[Dovecot] user xxx at mydomain.com doesn't exist

[G.Nau]

new setup of Dovecot 2.0.8 with postfix
local users are simply defined as Linux users named xxx, yyy et cetera;
IMAP access is fine with that users.
Postfix is delivering mails with two domains (to xxx at mydomain.com or
xxx at mydomain.org) and these are rejected by dovecot with 550: user
doesn't exist.

I've tested the possible usernames with "doveadm user"
xxx: okay
xxx at mydoman.com: "userdb lookup: user xxx at mydomain.com doesn't exist"
xxx at mydoman.org: "userdb lookup: user xxx at mydomain.org doesn't exist"

Dovecot2 wiki says "Dovecot doesn't care much about domains in usernames"
Okay, how to I get dovecot to ignore the domain part (or accept the two
defined domains) when mails are delivered from postfix?
Or do I have to configure Postfix to strip the domain part before
delivering to dovecot?

Regards
Birte Nauer