multiple domain rewrites

[Jim Werster <quixotal@yahoo.com>]

I am using potato.  How do I set up exim if I want to have 3 different
domains I want rewritten in my From: header?  I'm not talking about
what I set in .muttrc, I can change that per-user but it has no effect
on what exim does.  I have users that will use @yahoo.com,
@mailandnews.com and @freewwweb.com  for outgoing mail.  
I understand how to do this for one domain in exim.conf but don't
understand how to finely control  domain rewrites per-user.  

Also I have an pop e-mail account with 9 characters for the user name
but the login on my system is limited to 8. How do I rewrite the user
name properly for outgoing mail?  

Thanks,  Jim Werster