If you are using Outlook or Outlook Express you may occasionally find that mail is being sent repeatedly. There are a couple of possible reasons for this which we know of
This usually happens if you are sending a large message to a lot of local users (either as a list within Outlook or a VPOP3 distribution list). This happens because VPOP3 won't send back a 'message accepted' response to Outlook until it has distributed the message to all the local recipients (this is normal behaviour for an SMTP server).
If you send a large message to a lot of local users, this distribution may take a while. By default Outlook & Outlook Express have a timeout of 1 minute. With most email clients, if they time out, they will prompt the user, for instance, asking if they want to wait longer, or quit. So, the user will probably realise what is happening if the message is delivered repeatedly. However, Outlook will simply stop waiting, leave the message in the 'Outbox', and try it again later, without telling the user. This can mean that Outlook will try sending the message hundreds of times. Each time will actually succeed, but Outlook will not realise this because it is not waiting long enough for the response from the mail server.
You can increase the timeout in Outlook. Go to Tools → Account Settings → (account) → Change → More Settings… → Advanced, and change the 'Server Timeouts' value. (For information, the SMTP standard (RFC 5321) says that the timeout after sending a message should be at least 10 minutes).
After sending a message, Outlook & Outlook Express will move the message to the 'Sent Items' folder. On some versions of Outlook/Outlook Express, it may be that this folder becomes full, so that the message cannot be moved there. In that case, Outlook/Outlook Express will keep the message in the Outbox, so it will be sent repeatedly. In this case removing some messages from the Sent Items folder to free up some space should fix the problem.