well i've got a how-to guide on my local LUG website
http://tlug.dnho.net/?q=node/146
but by the sounds of things you want to edit a different PAM module, to be able to login on the machine with your domain password you'll want to edit the 'system-auth' PAM module. also i don't think the pop3 PAM module on the site above will be suited for your needs, so you'll have to look into PAM.
my suggestion is use the webmin pam module, it's pretty easy to get the hang of.