Letsencrypt and mail subdomains

Hello. I got problem with letencrypt. I make letsencrypt for hostname but when people connect to mail.domain.com they've got error about hostname missmatch.

The problem it's with connect to mail with SSL (i want to resolve that problem to connect from unsecured domain mail.domain.com, it's possible to add SSL for all only mail domains in server?)

What i need to resolve that?

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