One message sent but recipient receives 100's of copies

[#1]

Occasionally I send a message to one person. The message sticks in the Outbox for no apparent reason and the message is received many, many times by the recipient.  The last time the problem occurred the message included an attachment.
I am using Outlook 2003 with PcSecurityShield anti-virus software, all updates have been installed, I run Registry Patrol regularly and am not receiving any error messages or hints that anything is wrong.  Once I spot the message has not been sent I can delete it and the problem will go away although multiple copies will have been sent on that occasion.  It will be several months before the problem occurs again.  I did try uninstalling Outlook and reinstalling but that did not help.  My PC has been in use 4+ years so has had many bits of software installed and uninstalled.
Any ideas?

[#2]

Does your AV software scan outgoing emails? That might be taking longer than the Outlook timeout to confirm sending was successful so Outlook assumes it wasn't and doesn't delete the message from Outbox.
If outgoing emails are being scanned try turning the outgoing scan off. As long as your PC is clean that should be OK, and assuming mail is sent via the Plusnet relays messages are scanned in any case.
You could also try increasing the Server Timeouts setting in Outlook. This is done by clicking on
Tools then E-mail Accounts.
Make sure View or change existing e-mail accounts is selected and click Next.
Click an existing account to highlight it, then click Change.
Click the More Settings button.
Click the Advanced tab in the window that opens.
Drag the Server Timeouts slider to the right as required. I've set mine to 2 minutes.
Click OK, then Next and repeat for other accounts if required.
When done click Finish.