1. Execute the following query:
UPDATE `sys_profile_fields` SET `Check` = 'return (bool) preg_match(''/@mydomain.com$/i'', $arg0);' WHERE `Name` ='Email';
just replace mydomain.com with your trusted domain.
2. You can optionally disable confirmation emails and/or profiles approving since they should be trusted:
Admin Panel > Settings > Advanced Settings > Moderation > Auto-Confirm Profile Without Confirmation Email | Auto-Activate Profiles After Joining
