Other versions of Outlook will be similar, but with slightly different way of getting to the 'Account Settings' window). (For the purposes of this blog, we'll be using Outlook 2010. The default is 1 minute, so extending this to 5mins should do the trick (although under normal circumstances, of course, if an email is so large it takes longer than 5 minutes, it probably shouldn't be sent via email). The solution is to extend the amount of time Outlook will wait until it deems the sending to have failed. Whilst stuck in the loop, any subsequent emails sent, even if they aren't large, will also get caught up and get sent.
This process repeats, causing the same email to be sent multiple times.
However, the original email is still sending. If an email sending takes too long (slow network, large attachments, etc) then outlook seems to assume it failed and will try to send the email again. Outlook (various versions) seems to hit a problem when sending large emails. How to fix Outlook's 'Server Timeout' setting to avoid multiple emails being sent out Details Chris Andrews HowTo 04 April 2013
