The first version of SNI support for Mail is available in Plesk Obsidian: https://docs.plesk.com/release-notes/obsidian/change-log/#18-rc10
You need MailEnable 10.20+ (for Windows) or Postfix 3.4+ and Dovecot (for Linux) to test it.
There are cases when it doesn’t work (e.g., if you connect to mail.domain.com it will not work, please use domain.com in connection settings of your mail client) that’s why we don’t mark it as “available”. We plan to fix these issues soon.
As UserVoice staff cleaned up the most of twisted voices, I’ve returning this suggestion to open discussion.
Everyone, please continue voting for this feature if you consider it important.
—IBJoey den Hollander commented
Back to using C-panel. It's not feasible this way. My customers are not tech savvy enough to change their mail settings from mail.domain.tld to another domain because Plesk is still lacking this feature.