Recipient Email Address No Longer Valid

When to Use?

If your mailbox has recently been migrated to Microsoft 365, and you have received either of the 2 following error or warning messages when attempting to send an email, please follow the steps below to resolve the error.

Scenario One - Email Address Is No Long Valid

I've selected a contact to send an email to that I know is valid, but Outlook is showing a message that "We won't be able to deliver this message because the email address is no longer valid".

Scenario Two - Email Bounce-back "Your email program is using outdated address information"

After sending an email message to a contact I know is valid, I receive a bounce-back email message that says "Your email program is using outdated address information for:" along with a lot of cryptic text and IT related information.

Procedure

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this cached contact information may not update in time to point to the new Microsoft 365 email system, and like an out-of-date phone book, is instead still referencing the information for where the contact used to be in the old email system.

To resolve the error, there are two options:

1. Remove just the one contact you're noticing the issues with. (This is the recommended option.)
2. Clear the entire cache for all contacts.

Remove a Single Cached Contact from Outlook (Recommended)

1. Open your Outlook application
2. 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.

1. Open your Outlook application
2. Click File
   
3. Click Options
   
4. On the left-hand navigation menu, click Mail
   
5. Scroll-down 2/3 of the way to the bottom on the Mail options screen to the heading "Send Messages"
6. Under the Send Messages heading, click the button that says "Empty Auto-Complete List"
   
7. Click Yes to confirm emptying the Auto-Complete List
   
8. Close Outlook
9. Re-open Outlook
10. The next time you compose an email, after entering in an email in "To" field, click the Check Names button in Outlook