Using two server names on the same server

[misosoup]

Dear community,

we are currently at risk to lose control over our primary domain (long story...) and because of that we'll move all email addresses to a new domain.
I have created the new domain in the Zimbra admin panel and already tried just renaming accounts to the new domain - that works like a charm. Now the server name for SMTP, IMAP and HTTPS also needs to be changed, but it would greatly simplify the process if we could use both server names at the same time, so IMAP wouldn't immediately break for the users etc.

I've tried creating a new TLS certificate with the new domain as an alternative name to the old one, but Zimbra didn't accept that. The solution at least for HTTPS seems to be to use SNI: https://wiki.zimbra.com/wiki/Multiple_S ... _for_HTTPS
But as the article states, that would only work for HTTPS, not the rest. Has anyone ever been in a similar scenario and can give me a hint in which direction to look?

Thank you very much!