Hello Tim,
A few questions:
What's the conection between the site's?
Do jou have you own domain?
Can You use subdomaining? I think its nesseray.
If you have 2 are more site's you can use the sub domaining system.
The main site is psdata.nl and for each site u use :
amst.psdata.nl, eind.psdata.nl, gron.psdata.nl
The member get a e-mail adres with @.psdata.nl
If you use a intenal network to connect each server then you put the richt internal ip adresses in to your own DNS.
When you have you connection between your site's over the Internet you can also punt the right Ip nummers in to you own DNS server. Its almost faster then the outsite DNS.
For the incomming mail you use the sub domaining structure of your ISP to route the mail to his own server.
Your user administration is for every site on its own site server.
I hope you that you can use someting from this story.
Grtz
Johan