Virtual Mail User Question
 

What MTA/pop3d or imapd are likely to be supported by ISPsystem with virtual users?

Maybe you'd like to check this setup, and base it around that. It's very stable and reliable, I've been using it for awhile and uses mysql databases for its virtual user info check it out here --

http://workaround.org/articles/ispmail-sarge/

Until virtual mail users are supported, the e-mail system is pretty useless for servers hosting multiple domains. Once this is supported more and more people will start buying your nice software.

You are not right.
One of way use mail aliases
for example you need user@doamin1.com and user@doamin2.com
you can create user1 and add him mail alias user@domain1.com
create user2 and add him mail alias user@domain2.com
it is not good but it is solution.

thats sub-optimal, and still has some issues with usernames.

people should only need to enter usernames instead of using alias's.