SMTP Server crossing mailboxes

[Robert Canary]

Hmmm,  I know how it is happening, but I'm not for 
sure how to stop it.

I have about six different domains hosted on the 
mail server.  However, this only invloves two domains.

domainA.com and Bdomain.com

there is a virtual user set as user1 at domainA.com
and a virtual user set as USER2 at Bdomain.com

However, one can mix the usernames and the mail 
gose int the other mailbox.

Example:
user1 at domainA.com <-- correct.  If I send an email 
to user1 at Bdomain.com then it gets put into user1's 
mailbox at user1 at domainA.com.  The TO: header are 
intact and read "user1 at Bdomain.com"

How do I control this???

--
robert