My problem seems to be similar to the one in this thread
. He's using opensuse 10.2, the same as me. And I've confirmed this behaviour with multiple accounts. If there is an entry in /etc/aliases for the username portion of the email address (i.e. a system username, not an alias set up in ISPConfig), then the mail is delivered. If the recipient is only in virtusertable, then email originating on another server is processed fine, but if the origin and destination are both hosted on my server, it will fail.
Maybe there's something missing from the default configuration of opensuse 10.2's postfix package? Do your servers have an entry for fallback_transport (or something related) in /etc/postfix/main.cf?