There is a setting at the domain level to force users to configure 2FA authentication. Look for "Force two-factor authentication" on the General domain settings page, in the User/Alias card. Once this is enabled, users in the domain will be forced to configure 2FA the next time they log into the web interface.
When the domain setting is enabled, users will not be able to disable 2FA once it is configured. An administrator with the necessary privileges can reset the user's 2FA, which will require the user to reconfigure 2FA the next time they log into webmail.
The accounts page is probably the best place to determine which users have configured 2FA. Those who have configured 2FA will have an icon next to their username, as shown in this screenshot:
Andrew Barker
Lead Software Developer
SmarterTools Inc.
www.smartertools.com