Another option is to partner with Appriver, and have Appriver be the middle man. They support your client (sure you can support if you wish)..but they will too. And they will of course support you. They provision the server for you, they get it all setup for you....you just go in and add the user. But if you need any help, you can turn to them. They charge 2 bucks more/mailbox/month for their part. They bill your client, you get your 'mish through the back.