What are the special characters that can go in to the bare jid, is there any restriction by the jabber server

For e.g. can I have a bare jid as abc.def@gmail.com as my user id, so the complete JID would be abc.def@gmail.com@xyz.com. Are characters like @ allowed in the bare jid? (Where xyz is my host name). I tried the above thing it does not work with ejabberd 1.1.1. How can I know if there are other such special character cases under which does not work with ejabberd 1.1.1

Thanx in advance,
Ritesh