The 'Domains' tab in 'Company Settings' enables you to define the specific domains from which your users are permitted to send secure emails. This functionality helps maintain security by restricting users from associating personal email addresses with their accounts, ensuring that all secure communications are conducted through authorized company domains.
To add a new domain, simply click the ‘+ Add Domain’ button:
Type in your domain and then select ‘Add’ button to save your changes:
Tip: The general format of an email address is ‘local-part@domain’ and that only the domain element e.g. ‘beyenc.com’ is required.
After you’ve added your domain(s) select ‘Enforce on secure send’ - the button slides to the right:
When the 'Enforce on secure send' feature is enabled, you may see a 'Domain Violation' message next to a user's email address in the 'Users' tab. This message signifies that the user has an email address linked to their account that does not fall within your approved secure send domains.
Click on ‘View Profile’ against the users email address: 
From the profile, click on the ‘Email addresses’ tab.
This displays all email addresses associated with the account, clearly highlighting any that do not conform to your approved secure send domains and providing the option to remove them:
Note: When 'Enforce secure send' is enabled, invitations to new users will be blocked if the domain part of their email address is not in the domain list.