These issues may occur if one of the following conditions is true:
- You reply to messages that were migrated from personal folders (.pst) files or were migrated by using a third-party migration tool.
- The Outlook nickname cache (AutoComplete) contains a bad entry.
- The OfflineAddressBook is not updated or cannot be downloaded.
To resolve these issues, follow these steps:
- Reset the nickname and the autocomplete caches in Outlook. For more information about how to do this, click the following article number to view the article in the Microsoft Knowledge Base:
287623 How to reset the nickname and the automatic completion caches in Outlook
- If you reply to a migrated message, type the whole email address of the recipient again. If the Autocomplete feature tries to input the recipient name or address for you, click the "X" to delete the entry, and then manually type the whole email address again.