From here (
https://YOUR-PLESK-DOMAIN:8443/admin/ssl-certificate/list) you can set the certificate to use for the SMTP server, you can even set the Let's Encrypt certificate securing the plesk server if you want.
However, you will need to tell the users to use that domain and not their own.
So if you have your panel running on
https://pizza.cool.host:8443/ you should tell your users to use pizza.cool.host as the SMTP server domain when setting up their email (as long as the certificate you selected is valid for that domain).
This is a limitation by the SMTP server which can only handle a single certificate per IP.