Why don't you just set up one cert that you use for ssl / tls for imap, pop3, smtp and let all users access this one domain.
Your mail client does not care for which domain the mailbox is used for. The only "problem" is, that the user e.g. owns domaina.com and has to access his mailserver via yourssldomain.com which might "look" ugly but it work's anyhow. So from my point of view this is more a cosmetic thing.
All other multidomaincerts run into the problem that this must be supported by the client and the server. I did not hear about the fact that you won't run into the same problems like you do when trying to have a multidomain cert for webserver use (e.g. tls alternate names, where the client needs to handle this feature as well, otherwise it won't trust your multidomain cert anyway).