[Dovecot] Help implementing username_format in auth PAM driver

While configuring my server with dovecot I noticed that the PAM authentication driver does not support the username_format option as does the password file driver. This didn't seem too hard to implement so I through together a patch.

As you can see in the attached patch I only modify the username sent to PAM. Despit doing this I run into the domain lost issue(http://wiki2.dovecot.org/DomainLost). This prevents me from using the domain name in my mail_location config string. What I don't understand is why does changing the username string sent to PAM for authentication trigger this issue? Shouldn't dovecot continue to use the client supplied username as I am *not* changing it anywhere in my config?

Thanks,

Lee