This problem is related to the “the messaging interface has returned an unknown error” error message you get when trying to do something in Outlook.

The symptoms were the following:

- The message was trying to send but would not leave the outbox
- The message could not be deleted
- The message could not be moved out of the Outbox
- The recipient at the other end was receiving multiple copies of the email
- whenever trying to make any actions with the email an error appears

Like Outlook Express, Outlook 2002 has a 2GB limit on its file size. This means that the PST is either getting very close to 2GB or has reached its limit.

SOLUTION
The easiest solution is to reduce your PST file size to at least 25-50mb less than 2GB. Usually this involves deleting a bunch of emails like say mails in the Deleted items folder.

You can also try archiving your mail using the archive feature.

Another option is using the PST cropping tool but this can result in lost email.

The PST cropping tool is located here: http://support.microsoft.com/?kbid=296088