DNS SRV records
If the A record for example.com doesn’t resolve to your server you can still run a server for example.com by pointing DNS SRV records to your server. In fact, you should do this anyway, in the same way that your email will arrive if the A record for your domain points to the mail server, but MX records are still a good idea.
Assuming your Jabber server runs on a machine called jabber.example.com, you’ll want the following scary DNS records:
_xmpp-client._tcp 900 IN SRV 5 0 5222 jabber.example.com.
_xmpp-server._tcp 900 IN SRV 5 0 5269 jabber.example.com.
_jabber._tcp 900 IN SRV 5 0 5269 jabber.example.com.
You can check that they’re been set properly using this excellent tool, but it’ll probably take a while for the DNS updates to propagate. If you have the dig command line tool, you can also try
dig -t srv _xmpp-client._tcp.example.com
to ask your local DNS server for one of the SRV records.