iChat XMPP Federation doesn't work

Here's my situation,

I have 2 Leopard Server located at different physical location. Both are behind a third party firewall (SonicWall) with VPN and NAT enabled.

DNS services are provided externally by a third party, so DNS module is not running on the servers. Mail services are enabled and are working as they should on both servers.

Local users can connect to iChat server and establish a conversation between local users. But we can't add Buddy from the other Federated domain. The only mention that some form of connection attempt is made is this line from the iChat log entry;

... jabber/resolver [######] : [my.domain.com] resolved to [public IP:5269] (10800seconds to live)

All ports requiered by iChat are open on the third party firewall as per KB article # HT1507

- Third party DNS already points traffic of our sub domain to our WAN public IP on the firewall. I'm nor sure what else needs to be added to DNS record to have server to server ichat working.

- Do I have to run DNS service locally to have XMPP Federation to work?

If someone can point me in the right direction would be apprciated. The book "Mac OS Server Essentials second edition" did not help for my situation.

Thanks

Martin