[The Prohacker]

You might do it like an old UBB 5.xx hack did it, use the PM functions and convert them into mail handlers, have a php script, that would be setup in sendmail, so sendmail catchs all email for a domain, and sends them to the php script to be run at a command line, the email would be addressed to the users name, and be sent to their email addy. You could add extra things like pop3 mail box checking and all do that, might be easier, or harder than intergrating another mail client..