Two things cause this error: Either your anti-virus program is interfering with your Outlook, or Outlook itself is corrupted.
Try temporarily disabling your anti-virus program, then click on Send/Receive in Outlook. If everything goes well, then you know it's your anti-virus program. This usually happens only to older versions of anti-virus programs though, so you should contact your anti-virus program's support line for updates. Failing that, you can always uninstall your anti-virus program and replace it with
AVG Free.
If the error still occurs even with your anti-virus program disabled, exit Outlook. Look for a file named "pop3uidl.dbx", delete it (rename it so that you have a backup), then restart Outlook. That should fix things nicely.
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