I finally got a working solution.
After going over and over all the steps described in this HOWTO, I created a dir named "mydoma.in" in /home/vmail/ and a maildir for every user at this domain listed in the mysql user table with maildirmake <username> (without the "@mydoma.in" part as in the user table).
That allowed me to login and sent mails with thunderbird to foreign mail servers but I wasn't able to receive any mails.
In mail.info I discovered
postfix/trivial-rewrite: warning: do not list domain mydoma.in in BOTH mydestination and virtual_mailbox_domains
Google finally told me the answer: The entry for the domain in the mysql domain table can't be the same as set for mydestination. So i changed mydestination to mail.mydoma.in with
postconf -e 'mydestination = mail.mydoma.in, localhost, localhost.localdomain'
solved my problem.
Can anyone tell me if this is the way it's done or am I just using some kind of workaround?