Is it possible to post mail with the webmail.domain.com subdomain? is it possible to provide this as a standard with an automatic webmail dns record in each customer domain?
Yes this should do → https://cyberpanel.net/docs/2-ssl-for-postfix-dovecot/
And more detailed guide is available here → Achieve 10/10 Email score with CyberPanel! - Blog Posts - CyberPanel Community
I mentioned customer login mail entry.
for example.
webmail.domain.com/rainloop/index.php
mail.domain.com/rainloop/index.php
How to access from the customer domain name.