This extension is designed to preserve the email reputation of your server and its IP by ensuring that all outbound email is sent from an address with the same domain as the System-generated Mail Settings From Email Address configured at Administer > Communications > Organization Address and Contact Info (civicrm/admin/domain?action=update&reset=1).
If the server running CiviCRM is not authorized to send mail on behalf of a From address, perhaps because of a very tight SPF policy, it can lead to the server and / or its IP being blacklisted as a spammer.
This extension filters the From Email Address options provided to users as they are about to send an email. Only ones that have the same domain as the System-generated Mail Settings From Email Address are exposed, with the others suppressed.
It is fairly safe to assume that whoever initially configures CiviCRM will set up a From Email Address in the System-generated Mail Settings that is not barred from sending from CiviCRM's server. However, staff users authorized to send mail may inadvertently have a different domain used in their primary email and not realize using it will create problems. Similarly, they may create and use another From Email Address.