Hi DP,
You will need the certificate, similar to the one you are using on your web server. You even can use the same certificate you are using on your web server.
But, if you do not certificate which covers all subdomains of you domain name, then you may want to use a certificate for subdomain that points to your server.
In all cases, import your certificate into the certificate store and then point to it from your mail server, from Security - Server Certificate.
Archie