[exim] give me advice for account issue

im using Exim 4.63 on debian etch.

im setting up exim on the system required some virtual domains, and i
have to set up this system with next requirements.

  1. accounts having same localpart and different domainpart is
     completely different account.
     for example,
          user@???
          user@???
     this two mail address have to be independent.
  2. accounts have to be authenticated with their mail address.

generally, virtual domain is set up with aliasing.. but if i use
aliasing, i can not have different accounts with same localpart(and
different domains).

so, i decided to use mail address as account name, but this bring me
another problem such as qualifying (user@??? ->
"user@???"@qualify_domain.com, I can not send mail such these
having "Sender" address to another MTA ).

so, i wanna know how to force to set $localpart and $domain variable to
the value from "account"
or.. im still afraid of using "@" in account name, so if there is
another good way for this issue, please give advice

thanks.