We have email-only subscriptions on our server. The DNS is hosted on Plesk, which helps SPF, DKIM and the webmail certificates work. The DNS then points the website to a different IP address.
One recommendation is that your external website is hosted on
WWW.domain.tld and that you have domain.tld hosted on your Plesk server. You can then easily set up a deep-linking redirection catch-all webpage. The reason for this is that then domain.tld (which is typically used for email client settings) is pointing to the host which runs the email server.
It's a bit of a fudge... but it works!