The error may occur when sending to an individual who has been migrated to a new e-mail server. The issue could be caused by the AutoComplete feature, the Global Address List (GAL), and/or Contacts.
Autocomplete: Outlook caches the previous address 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, use the arrow keys to select it and press the Delete key.
Outlook 2013 no longer uses an nk2-file for caching but stores these addresses in the Suggested Contacts folder in the Outlook mailbox itself. When you upgrade to Outlook 2013, the nk2-file is automatically imported upon starting Outlook 2013 for the first time.
Outlook 2010 steps:
- Exit Outlook
- Search for the file Outlook.NK2 and rename it to Outlookbkp.nk2. (NOTE: When you search for the file, you will need to search in hidden files and system folders and display file extensions). The file will automatically be recreated the next time you launch Outlook.
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The GAL may also contain an out-of-date entry that may cause the error. If you are using Cached Exchange Mode and never close out of Outlook, the GAL will be outdated. 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An Outlook Contact entry containing the outdated target/routing address will need to be updated.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., firstname.lastname@example.org), you should update the entry to use the alias (email@example.com).