[imp] email@address -> id -> courier login

[Manou]

Hello,
I use Horde: 2.2.5 and IMP: 3.2.5. under debian
I want tu use it on POP3 service on courier-pop with mysql.
Now, courier-pop is configured to authenticate from id, not email. And I 
don't want to change it to email.
But from webmail, I want users to log from email address (because of 
multi-domain).
So I think to hack IMP to select the id of the email at address before 
sending this id to courier-pop for authentication.

Would someone what file in IMP can I modify? Where can I find docs for 
it? I already read http://dev.horde.org/api/imp/ but I'm not really good 
in php.

Thanks in advance.
Manou.