Solved! Go to Solution.
AltaLink. EWS > properties > Services > Apps > Email > Setup > Security
Or you can use the Domain Filtering option directly above there to block specific domains or to allow only specific domain(s)
VersaLink - EWS > Permissions >Guest Access > Edit > Device user Roles
Custom Permissions > Setup
Pick Email in the list and scroll down
If domain fioltering can be done on VersaLink, it would have to be globally, not just email, and I have never tested it, but in theory:
EWS > System > Security