To use xmpp (jabber) within a domain domain.com (not the default domain myserver.mydomain.com) i had to add entries to the dns of domain.com like this in bind9:
With this, a jabber id like firstname.lastname@example.org can be used, but i had to make an alias email@example.com so that the server knows about user (without domain) for authentication. I didn't get any xmpp client to use firstname.lastname@example.org as username for authentification.
_xmpp-client._tcp.domain.com. 86400 IN SRV 5 0 5222 myserver.mydomain.com.