Error Message
We won't be able to deliver this message to XXX, because the email address is no longer valid.

Description
These errors can occur because your Outlook application saves—or "caches"—information for email addresses that you have sent messages to in the past. After a mailbox move, this cached contact information may not update in time to point to the new Microsoft 365 email system, and like an out-of-date phone book, is instead still referencing the information for where the contact used to be in the old email system.
Resolution
To resolve the error, there are two options:
-
Remove just the one contact you're noticing the issues with. (This is the recommended option.)
- Clear the entire cache for all contacts.
Remove a Single Cached Contact from Outlook (Recommended)
- Open your Outlook application
- Click New Email

3. Start typing the name or email address for the contact you'd like to send to
4. You will notice a pop-up below the "To" field that contains the contact's information
5. To the right of the contact's information, there is a small X - Click this X to remove the contact

6. The contact is now removed from the local cache
7. Start retyping the contact's email address, but don't click anything until the complete email address is entered
8. Once the complete email address is entered click the Check Names button in Outlook

9. After clicking Check Names, the email address and contact will now be re-saved and cached in your Outlook application
10. You are now able to resend your email message to that contact
Remove All Cached Contacts from Outlook
-
If you would like to preemptively clear all cached addresses from Outlook, you can do so with the following steps.
-
Open your Outlook application
- Click File

- Click Options

- On the left-hand navigation menu, click Mail

- Scroll-down 2/3 of the way to the bottom on the Mail options screen to the heading "Send Messages"
- Under the Send Messages heading, click the button that says "Empty Auto-Complete List"

- Click Yes to confirm emptying the Auto-Complete List

- Close Outlook
- Re-open Outlook
- The next time you compose an email, after entering in an email in "To" field, click the Check Names button in Outlook

Additional Information
For the most efficient support experience, please submit a ticket using the "Get Help" button, or call 531-622-2900.