correct, the cert is for the domain, however in your CL posting you said you needed multiple domain virtual hosting. The cert is specific to a mail server hostname. It really depends on how you want the actual MX record and outgoing mail server setup.

So if you're hosting three domains: domainone.com, domaintwo.com, and domainthree.com, you have a few options.

You can set the MX of all three domains to mail.mymailserver.com, and have a single cert. However, that means all of your end users will have to point their IMAP/POP clients and outgoing SMTP server settings to mail.mymailserver.com, instead of mail.domainonce.com which might seem more logical to them. It also means that your outgoing mail headers will show the mail.mymailserver.com hostname, and you'll need to setup the right SPF records to avoid being flagged as spam due to domain mis-match.

Or you can set the MX for each domain to mail.domainone.com, mail.domaintwo.com, etc...

It really depends on your needs, and the needs of the client/users who are checking and sending mail, and how public you want the fact that all the domains are on the same server.

Feel free to follow up with me via e-mail. devon@digitalsanctuary.com