Username with different @domain does not work

I have the requirement that the user name (login name) of each e-mail address of the registered user must comply. Example: the chat server has the @ domain1.de, the logins of users userx@domain1.de; userxy@domain2.de, userxyz@domain3.de etc.

My problem: Some Jabber clients like PSI can use only user name with the domain name of the server (in this example @

domain1.de) for registration.

Question: What can I do, the user logins - regardless of their default (Server) domain - eg assign an alias?

Best regards,
Arnim