Currently, when a website (e.g. mydomain.com) is created, the mail server cannot be a mail. subdomain by default without going through a number of lengthy steps. That is, creating another website (e.g. mail.mydomain.com) and assigning that mail. website's SSL to the mail server. This is a lot of work and most web panels including Virtualmin cut out this work.
If this long process is not followed, then the mail server's SSL is tied to the root domain (i.e. mydomain.com). Most mail clients--including Microsoft Outlook and Mail for Windows 10--auto configure when supplied an email and password to the mail. subdomain and will display errors if the mail server's SSL is not linked to the mail. subdomain.
To make things easier for everyone, please add an option so the mail server has its SSL configured for a mail subdomain when creating a website.