Emails keep sending, what do I do?

This has been a problem for over 6 months.

·         When I send an email to multi recipients (work or home) with an attachment, the email gets stuck in the outbox of Microsoft Outlook and continues to send the email until I delete it from the outbox.  The recipients are receiving the email in excess of 50 times.  

Needless to say, my recipients are getting full inboxes and getting very annoyed.

I have actioned the following:

·         Long time user of PC-cillin, + running of Trend Micro House Calls scanning.  I cannot find any solutions with this area or settings

·         Microsoft Outlook: can’t find any settings that are unusual.

·         Use Broadband Telstra @ home and Broadband Australia Star @ work.  Problem happens at both locations.

·         It happens on both my personal email address (with TPG) + my work email address (with TPG)

·         Windows updates regularly as per standard settings.

·         2 other computers (out of 7) at my office also do this.  1 is only a couple of months old and did it on the 2nd day of use.

Please Help!