AppleShare IP (ASIP) can handle email for multiple domains as long as you don't need a specific username to go to different people. That is, user1@domain1.com and user1@domain2.com have to be the same person.
You do this by setting up entries in DNS so that other SMTP mail servers will know that your ASIP Mail server is handling all mail for the given domains. Basically, you're setting up DNS so that mail.domain1.com and mail.domain2.com point to the same server.
Given:
ASIP Mail Server's IP Address: xxx.xxx.xxx.xxx
ASIP Mail Server's domain name: mail.domain.com
Set up a DNS MX-List for:
domain1.com MX
10 mail.domain.com
100 mail.your_isp.com <---- note this line is optional
domain2.com MX
10 mail.domain.com
100 mail.your_isp.com
domain3.com MX
10 mail.domain.com
100 mail.your_isp.com
AppleShare IP Mail will handle mail for any domain with an MX-List where the ASIP Mail server is the lowest priority alternative choice in the MX-List.