Yes, abc.com is the domain name for virtual hosting and it's allso the domain for our three servers. But we solved the problem by creating theese mail accounts on the mysql server and making a forward to real users who should read root/postmaster/etc mail and it seems to be working quite well.
The quota is another case. No matter how big I set the value in the database we get:
postfix/smtpd: NOQUEUE: reject: MAIL from aaa.bbb.org[xxx.xxx.xxx.xxx]: 552 Message size exceeds fixed limit; proto=ESMTP helo=<aaa.bbb.org>
... and the server disconnects. I tried to put bigger quota on both sending and recieving users (the sending one really should not matter?) both on the same server.