Hello @David Beroff,
In fact both of them will work, because any domain or subdomain pointed to your server IP can be used a SMTP address.
But it's highly recommended to use SSL/TLS connection between your mail client and your server, and you will probably get issues or warning by using mail.domain.com or webmail.domain.com.
Because Plesk only support configuration of a single SSL certificate to secure the mail server. So, if you choose mail.domain.com, and use Let's Encrypt to issue a SSL certificate, it will work for this domain, but for all other domains (eg mail.example.net), you will have warning about the SSL certificate.
The best solution is to use the same IMAP/POP/SMTP address for all sites hosted on your server and to issue an SSL certificate with Let's Encrypt for this address.
I recommend to my customers to use the server hostname to access to the Plesk web interface and as mail server address. For example, if your server hostname is plesk.yourdomain.tld, you can use it as IMAP/POP/SMTP host for all sites on your server.