Microsoft Outlook has a feature called auto completion. It is a cache that stores names and email addresses so you don't have to type it each time you write an email to the same person. Sometimes, the auto completion cache may contain outdated information. If this is the case, you can easily remove individual addresses from the cache file.

This can be accomplished by highlighting the incorrect entry when it’s presented in the suggested names list (use the arrow keys to select an entry). Press the delete key on your keyboard and the entry will automatically be removed.

Sometimes this file can get corrupted and cause mail to sit in your outbox or you get bounce back messages from people that are in your auto completion file. In this case, it may be better to actually reset the whole cache file. Below are instructions on how to accomplish this.

1. Open Windows Explorer.

2. From the Tools menu, click Folder Options.

3. Click the View tab.

4. Select the Show hidden files and folders option if it is not already selected.

5. Click OK.

6. Click Start > Search.

7. Click All files and folders.

8. In the All or part of the file name field, type *.NK2.

9. Select All Hard Drives from the Look in field and click Search.

10. Right click the .NK2 file under the correct profile name and click Rename.

11. Rename the file to [profilename].bak and press Enter.

12. Close any open windows and restart Outlook.