Login with maildomain-alias address

Hello everybody,

I'd like users to be able to connect to the mailserver using either 
their "real" username or their username in an alias maildomain that is 
defined in vexim.

To make myself clear:

maildomain: example.org
alias-domain: example.com

This means that user [email protected] gets mail for [email protected] 
and [email protected] (works great).

Now to make things easier for him (because example.com is used 
everywhere else = webserver, etc.), I'd like him to be able to logon 
(and also send mails) using [email protected] _or_ [email protected].

Is anybody doing this? For exim it is probably just a litte change or 
two in the config-file(s).

Best regards, Robert