User Issues

[Vincent Bell]

Hello I am having an issue here,

I have a server with several different users and domains. (ie, test.com, 
raretest.com, judgeelect.com) all my mail boxes are on the same server. 
When someone from any of the domains regardless of their alias login 
they get the raretest.com alias as what there sending from and its not 
reading their IMAP account information from the server? Any suggestions.

Details:
mbox: - adsupport
alias - support at domain.com

when they login, they get --> adsupport at domain.com
when they recive they get support at domain.com
when they send they get adsupport at domain.com

why is that happening, why is that when the user logins in, its creating 
their own alias and not reading the imap server?