Mail DNS strange behaviour

[Vartan]

Fisrt a few words on the config :
a tenth of iMac running OS X, connected to a Mac OS X Server. The home dirs are on the server and the apps on local drive. NetInfo setup so that anybody can log in anyMac.

I've setup an internal mail server (IMAP only) on the server, we're connected to the Net but have no DNS entry on the Net so my DNS is only local and caching.

When I go in Server Admin to add entries of possible mail servers, I found that one of them have been erased and that Server Admin refuse me to add some entries like the one that is my server name. I don't understand why as far as that entry must be the one used for internal mail running only IMAP. When I say it refuses me to setup the domains that does not mean it show me a message but if I close the window and reopen it to see if my settings are recorded I see that it doesn't.