The mail server's main is the hostname, which has SSL.
So when you setup email accounts, for the mailserver you would use that hostname (e.g. srv1.servername.net)
Clients like Thunderbird should follow the A record, and find this.
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Or you can try how we use to do thing, back when we had IIS & IMail.
Create an A record for mail.domain.name
In your MX record you will create, MX 0 mail.domain.name
On the CWP you are using for mail, create the domain name of mail.domain.name.
This will allow you to create a SSL for mail.
Only trick when they login to webmail, it will be user@mail.domain.name, instead of user@domain.name.
But I'm curious, if you have a CWP server running with the website, why not use that server for email?