email users

Hi,

I don't even know how they did this, but I have a client with a webmin server, and we are moving it to a new virtualmin server. We have an issue with the email users, and this is only a mail server

1. there are 4 domains for mail, which I don't know how they got webmin to virtualize.
2. there are 400 accounts. It looks like they may all be server level users, but I don't have root at the old server to see.
3. They use a user, and password, without the domain name in any way on the login to the mail accounts.
4. Virtualmin does not make them this way.

Is there a workaround so we don't have to make everyone change their logins?

So a mail account would look like:

email address: howard@execvps.com user: howard password: password

Any ideas?