Cleaning up Your Outlook Contacts Auto-Completion Errors

Here's the Lowdown: When you receive a bounce-back email with the error #550 5.1.1 RESOLVER.ADR.ExRecipNotFound, try clearing cached entries from your Nickname list or deleting your Nickname Cache file.

If you are having problems with a single recipient, you can delete the individual cached entry.

To Delete a Single Cached Entry

Open a New Message window in your Outlook account.
From the File menu, select New- Mail Message.
Type one or more letters of the recipient's name or email address. The cached entries automatically display in the Email list.
Select the entry you want to delete with your arrow keys, and then press the DELETE key on your keyboard.
The entry is removed from your Auto-completion Cache.

If you are having problems with multiple recipients, you can delete your Nickname Cache file.

To Delete the Cache File in Microsoft Windows XP

(Optional) Exit the Outlook program.
Start Microsoft Windows Explorer.
From the Tools menu, click Folder Options.
Go to the View tab, and from the Advanced Settings menu select Show Hidden Files and Folders, and then click OK.
Click Start, select Search, and then click All Files or Folders.
In the Search Companion area, type .NK2 in the File Name field.
From the Look area, select your local hard drive, and then click Search.
Right-click the .NK2 file for which you want to reset the name, and then click Rename.
In the File Name field, type profile name.bak, and then click Enter.
Exit Windows Explorer, and restart Outlook.

To Delete the Cache File in Microsoft Windows Vista

(Optional) Exit Outlook.
From your the Start menu, select Search.
Click Advanced Search.
Select Include Non-Indexed, Hidden, and System Files.
In the Name field, type .NK2.
Delete .NK2, and then restart Outlook.

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