A common use case is a company or organization that has multiple domains or domain aliases that are "functional" but not the primary domain for an organization. This is common when an organization has acquired another company or if there are subsidiaries that have domains that are similar to the primary domain.

A Trusted Mail Domain is exempted from a number of our anti-phishing protections.

As such, only fully trusted domains from within your own organization should be added to this list, do not use this feature to "whitelist" a 3rd party domain.