IMS can be set to accept multiple domains and multiple MX records can point the IMS.
IMS routing can accept many inbound domains and by assigning the proper SMTP addresses to the users, you can effectivly host many internet domains on one exchange server.
So lets lay out an example:
you have 2 domains a.com and b.com and your Exchange servername is exchange.isp.com
you will have an MX record that points to exchange.isp.com for each domain
a.com mx 10 exchange.isp.com.
b.com mx 10 exchange.isp.com.
In the IMC, Routing section you set a.com for an inbound only domain and b.com as an inbound only domain.
To identify the individual users:
Lets say Fred and Wilma have accounts on a.com
and
barney and betty have accounts on b.com
in fred's address listing in fred's exechange account, you will have an internet address of fred@a.com and in wilma's account you will have an internet address of wilma@a.com (That must be the only internet address listed unless your going to allow for aliases for each user)..
The same for barney and betty...barney@b.com and betty at b.com.
Once everything is set the mail follows the following path...
Sender write mail to wilma@a.com
looks up mx record, gets host address for mail, sends mail to the reciever at exchange.isp.com. Since a.com is an inbound domain, Exchange looks for an internal address that matches the wilma@a.com address, finds it and delivers it to the mailbox.
I realize this is a lot of info, please let me know if you have any questions.