SSL for email server - which domain?

This article describes setting SSL for postfix & dovecot.

A requirement is that the RDNS for the server’s IP is the same domain name used for the mail server. Since I have to pick from a drop-down of available domains in Cyberpanel, that seems to imply I have to create a ‘website’ in Cyberpanel and also setup RDNS for that domain?

Is it correct to assume that I will be picking a domain name for my server which can be the RDNS and also have that a website in cyberpanel even if it is only dedicated to email?

I use the main domain for the mail ssl, server hostname for just the hostname ssl works for me but depends on requirements such as how many other domains are being hosted.