Recipient Email Address No Longer Valid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is valid, but Outlook is showing a message that "We won't be able to deliver this message because the email address is no longer valid".

Outlook warning message that a contact or email address is no longer valid

Scenario Two - Email Bounce-back "Your email program is using outdated address information"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message to a contact I know is valid, I receive a bounce-back email message that says "Your email program is using outdated address information for:" along with a lot of cryptic text and IT related information.

Microsoft Outlook bounce back message example when an email is sent to an invalid contact.

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this cached contact information may not update in time to point to the new Microsoft 365 email system, and like an out-of-date phone book, is instead still referencing the information for where the contact used to be in the old email system.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
    Outlook application main ribbon with the New Email button highlighted
  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
    New Outlook email message with the a contact autocomplete pop-up shown, and the X used to deleted the cached contact highlighted.
  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
    New Outlook email message showing an email address in the To field and highlighting the Check Names button on the ribbon.
  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
    Outlook application with the File button highlighted
  3. Click Options
    Outlook application file menu with Options button highlighted.
  4. On the left-hand navigation menu, click Mail
    Outlook application Options menu with the Mail menu item highlighted.
  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"
    Outlook application Mail Options screen with the Empty Auto-Complete List highlighted.
  7. Click Yes to confirm emptying the Auto-Complete List
    Outlook application Empty-AutoComplete List confirmation dialogue box with the Yes option highlighted.
  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
    New Outlook email message showing an email address in the To field and highlighting the Check Names button on the ribbon.
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