How to secure mail when domain (page web) is on another server?

Hi,

When the domain (web page) and mail are on the same server, after generating a certificate for the domain, I can protect the email using the domain name as host.

However, if I have a domain (web page) on another server and the mail is on Plesk, what do I need to do to protect the mail?

I generated certificates for webmail.domain and mail.domain, but neither allowed me to make an SSL connection to protect the mail.
Can help me?

Thanks.