How to assign separate SSL certificates for different domains on a local mail server in Plesk?
This functionality is not yet implemented in Plesk. If you would like to see this feature in Plesk, please vote for it on Plesk UserVoice:
The top-ranked suggestions are likely to be included in the next versions of Plesk.
Currently, Plesk allows to install only one SSL certificate for a mail server.
As a workaround, it is possible to purchase a SAN (multi-domain) SSL certificate.