I posted a question if this was at all possible
here but judging by the lack or replies I'm guessing it's not.
So please consider this my official request for this.
I'm guessing that in most cases, RC is creating the wrong default identity for new users since it either uses $rcmail_config['default_host'] or virtusertable. In both cases the chances this actually creates a usable identity are slim. For several reasons.
So my request is to force a user to create a new identity the first time he logs in, or at least have him confirm the identity created by RC.
The way it is now not only utterly confuses a lot of users, but poses a serious security risk as well because if a user doesn't change his identity, it exposes information you really don't want exposed (especially when a mail domain is a virtual domain, and the local part of the email address is an alias. RC will actually expose the real server name and a real username - hence security risk).