[horde] aliases

[Gwanûn]

hi 
I have a problem using imp3 and exim :-( on my server I host two domains (for example foo.com and bar.com)
well, I set up exim to use different aliases files for the two domains (/etc/aliases/foo.com and /etc/aliases/bar.com).
I did this because I want webmaster at foo.com and webmaster at bar.com. the two users are webmaster-foo.com and webmaster-bar.com. now my problem is that I have to login to imp3 with 'webmaster-foo.com', but I want to login with webmaster or webmaster at foo.com. how can I do this? does something like an aliases file for imp3 exist? or should I use
virtualhosts? btw, I've spent a lot of time in searching the mailinglist on google, but I didn't find a solution for the use of virtualhosts with imp3 :-(((

thanks in advance
gwanun