Sometimes a setting changes on a mail server that changes the way email routes to a person. The email address of the person is the exact same, but the path to their email may have changed internally on the mail server. Maybe the mailbox was moved to a different storage drive, or maybe a different server.
The most common errors are “Your message did not reach some or all of the intended recipients.” or “Delivery has failed to these recipients or groups.”
When this happens the email contact that you have saved in your address book no longer works. It shows the same email address, but since something changed on their mail server, the email address you saved for them no longer works.
Luckily, the fix is simple! Delete the email address for saved contact, and type their entire email address manually.
If that doesn’t work, you’ll often need to get their IT support involved. There could be something else on the mail server that must be cleared out.