Problems with "Discard Groove messages from unknown contacts"

Under the Options tab of Groove Preferences, there is an innocuous looking option, Discard Groove messages from unknown contacts. While this option is useful in some situations, it is important to understand the problems that may result from using it. All of these are well documented in the Help, but are not obvious from the UI.

Obviously, when this option is enabled, messages from unknown contacts -- that is, Groove accounts that are neither in your known contacts list nor verified by your Groove domain -- will be discarded. This is done silently. Because accepting an invitation relies on an exchange of confirmation messages, you will not be able to accept an invitation from an unknown contact either -- not even if the invitation is saved to a file, and made available to you via email, a web page, or a file share. Similarly, unknown contacts will not be able to accept invitations from you. Because the messages are discarded silently, you will have no indication of what failed.

The Groove Known Contacts or Known Groove Users list is assembled from a number of sources. (For details, see "A Groove contact may still appear in the Known Groove Users list after you delete the contact" at https://support.microsoft.com/kb/917195.) One of these is workspaces. For that reason, if you have your account on multiple computers, and have workspaces that only exist on one of the computers, a contact that is known on one computer may be unknown on the others, leading to apparently inconsistent behavior.

Because of these complications, this option is recommended only for short-term use. If you are in a situation where you continue to receive too many unwanted messages from unknown users, see the Recommendations section of the Help topic "Restricting messages from unknown contacts" for guidance on moving to a new identity. If you had intended this option as a security enhancement, see the Communication Policies options under the Security tab for Groove Preferences instead.