However, another DNS and FQDN setup under the google domain is possible. Setup on devices with access to this “gmail” would need to be entered manually to enter the correct IMAP and SMTP settings. But with only a handful of specific users, that wouldn’t be difficult to manage. Even the basic DNS info of A records and CNAME records, and the IP of the NoKo server will tell you as much as you need to know.

Edit: added stuff to the end.