Let's Encrypt certificate for Mail server enable for all domains on server.
Currently this funciton is useless on a server with multiple domains, because the certificate is not recognized for domains different from the server one.
Haven't checked it, but I suspect Plesk does not provide the correct ssl certificate for ssl-enabled domains (the same problem with mail/ftp/login:8443 etc).
If a domain has a properly configured ssl, it should be used for securing web/mail/ftp/login. At this moment, we are having invalid certificate/server name mismatch warnings everywhere.