I have Outlook 2010 on my PC running XP. When ever I send a message with an attachment it stays in the outbox and sends repeatedly. It even maxed out my bigpond account ! I tested sending it to my hotmail account and it sent it 22 times before i stopped it and manually deleted it. I have changed mail profiles so it sends sent mail to another account but nothing works. I never had this problem with 2007 Outlook ! It doesnt seem to matter on the size of the attachment. I have tried with 1mb, 2mb and 4 mb. All the same results.