Catch-All Mail-flow (Transport) Rule
Before we can create the rule, we need to have a group defined that consists of all users with a mailbox license (Users with Exchange mailboxes). This needs to be a dynamic or automatically populated group so that whenever a user becomes unlicensed, then they will not become an orphaned member of the list. Conversely, we certainly want all newly licensed users to dynamically become part of this list without requiring an administrator to manually perform this task.
These group members are used as an exception within the rule. All mail will be delivered to the target email address, except those that have been addressed correctly, which will be delivered properly.
To create this group, we would create a Dynamic distribution list in the Exchange admin center (EAC).
- Go to office.com and login with your work or school account
- In the navigation bar on the left, choose the Admin icon.
In the lower left, choose Exchange from the navigation menu. If not shown, choose, “Show all”.
In the left bar menu, pick Recipients and then Groups.
Add a group.
Choose a group type: Dynamic distribution.
Name your group and describe it, then Assign users
Edit settings: Group email address
Review and finish adding group
Now we can create the transport rule.
Still in the EAC, choose Mail flow > Rules > Create new rule…
Name your rule
Apply this rule if… > The sender is located… > Outside the organization
Do the following… > Redirect the message to… add – > user or group
Scroll down to More options…
Except if.. > add exception
The recipient is… > is a member of this group
Now is the time to choose the ‘Dynamic distribution list’ that we created in Step 1
In the example below, I called it ‘C365 Dyn Dist Grp Exch Users’
Save > Test