I have my webmail configured like that!!!
go to /home/httpd/html/horde/imp/config/servers.php y servers.php.dist
I enter the domain and IP of my mail server (is on windows 2000, sorry about that)
then I enter in mail options with server-manager, look for delegate server (e-mail retrieval), and enter the IP of the mail server, then in internetprovider's smtp enter the ip of the mail server (other e-mail options). I set up sme mail server as private, and port forward to the other mail server (I did this because my mail server is on the local LAN). And for last, on each user I choose e-mail delivery the option "forward e-mail to address below" and in forwarding address I enter the IP of the mail server.
I did all of this because I wanna use webmail with my mail server (on Windows) and all the services (IMAP, POP3, SMTP), with my local users and foreing users too.
I don't know is this what you want, but could be ok for someone looking for a solution like this!!