It's probably nothing to do with your PC or Outlook Express.
As e-mail messages pass along a stream of servers, each receiving server returns an 'acknowledgement' back to the previous server to show that the mail has been received. If no acknowledgement is received, the sending server repeatedly re-sends the mail until it gets an acknowledgement. If there's a problem with the return path from the receiving server, the acknowledgement won't get through even though the mail already has - so the mail is sent again and again until the acknowledgement finally makes it back to the sender. (If no acknowledgement is received after a very large number of attempts to send the message, you'll receive a 'bounced mail' message - even though the recipient might have 50 copies of it!).
So, as I've indicated, the problem was probably related to something along the path that the e-mail took, rather than at your end of things.
Chris