ichat&mail server dns configuration

hi all there's one thing I don't understand about iChat and mail server DNS configuration. I have configured a new zone domain.com with a configuration like that (in synthesis): - MX: mail.domain.com - A record for mail.domain.com - A record for ichat.domain.com is it possible that I cannot use user@domain.com as email address and iChat account but that I have to use user@mail.domain.com and user@ichat.domain.com? is there any configuration I'm forgetting? thanks in advance for your help