Email to coworker bounces with error
This can occur when you change your email hosting from one server to another server. This could be from an in-house Exchange server to a cloud based server or from one cloud based server to another server.
The reason for this is that when you start to type the name of your coworker Outlook shows you the full email address in a drop-down menu. While the address will look okay to you ( like email@example.com) there is really detailed information stored behind the scenes that ties this name to the OLD server which is no longer relevant.
When you see the full address presented in the drop-down hover your mouse over it and you will see an "X" to the right of the name. Click the "X" to delete this memory record and clear your computers memory of this item. Then, type in the full email (like firstname.lastname@example.org). This will now be free of the pointer to the extinct server and your mail should not bounce back. You should only need to do this once for each of the people in your organization that you send to.