Before I started hosting my own accounts I had aliases called "webmaster" and "admin" and "info"... and im sure on their box they had a user name "webmaster" etc.
I've found a problem in the past that _breaks_ all aliases, I guess this could be it.
For some reason, _sometimes_ the GUI will let you create an "admin" alias for a virtual site. I know because I used to create them by default for my customers.
Then I found out that ALL the aliases "admin", "postmaster" (and I think "abuse" too) are routed to the "admin" user. You _can't_ create this aliases on a virtual site (in theory at least).
BUT if you manage to create the "admin" alias (as I did), you will break all aliases until you remove it. This is what will happen:
whatever@virtualsite will appear in whatever@mainsite (if "whatever" user exists). i.e.: if the main site has a user called "joe", he will start getting all mail directed to joe@ANYvirtualsite.
From what I remember, once this happens you're not allowed to create an "admin" alias anymore, but I'm sure I had created it for about 10-20 sites. I had to go site by site deleting the admin alias until everything started to work again.
If anybody can shed some light on this, I'd appreciate it. Best regards, Jorge