The error may occur when sending to an individual who has switched to a different email system or changed their name/email address. The issue could be caused by the AutoComplete feature, the Global Address List (GAL), and/or Contacts.
Autocomplete: Outlook caches the previous address used and needs to be cleared before the new correct Office 365 addresses will be added to the autocomplete list.
To remove a single address from the AutoComplete list, create a new message, start typing the recipient name, use the arrow keys to select the bad address and click the X next to the address or press the Delete key. .
Global Address List (GAL):
The GAL may also contain an out-of-date entry that may cause the error. If you are using Cached Exchange Mode you may need to update your address book. The Offline Address Book is updated daily, but if an e-mail client isn't closed, the updates will not be applied. Here are some ways you can ensure that your GAL has been updated:
- Close out of Outlook overnight
- Turn off Cached Exchange Mode
- Close and re-open Outlook
- Turn on Cached Exchange Mode
- Manually update the address book by going to File | Account Settings | Account Settings
- After this a window should appear, go to the top right tab that says "Address Books"
- This will give you the option of Adding or Changing the address books that are synced with your account.
An Outlook Contact entry containing the outdated target/routing address will need to be updated. If a contact entry contains the routing/target address (e.g., email@example.com), you should update the entry to use the alias (firstname.lastname@example.org).