User Issues

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@domain.com

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