[imp] IMP 3.1 Hook Needed

[Boz]

Ive read the Faqs I know my problems answer, I need to make a hook in
config/conf.php and am looking for some kind soul that can whip up the
code as I havent been able too.

Scenario is as follows:

1 mail server with address mail.mydomain.com

Serves multiple domains, figures out which domain and account to go to
by username. 

Username is formatted by user%domain.tld (yes the % is acurate not an @) 
So I enter user%mydomain.com and my password and it logs into
mydomain.com fine and when I enter user%anotherdomain.com and my
password it logs into anotherdomain.com 

I can read mail no problem with this setup but when I go to compose or
reply I end up with user%mydomain.com@serverdomain.com or
user%anotherdomain.com@serverdomain.com in the From: field depending
what was in the username login (serverdomain is configured in servers) 

Hope this is clear enough.

Someone has got to have the skill to write the hook to fix it so that
it just has user@mydomain.com (drop the % and replace with @) in the
From: field when I enter user%mydomain.com in the login and have
user@anotherdomain.com in the From: field when I enter
user%anotherdomain.com in the login. 

Thanks to anyone who can help.