Frequently Asked Question

Outlook (various versions) seems to hit a problem when sending large emails. 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. However, the original email is still sending. This process repeats, causing the same email to be sent multiple times.

Whilst stuck in the loop, any subsequent emails sent, even if they aren't large, will also get caught up and get sent.

The solution is to extend the amount of time Outlook will wait until it deems the sending to have failed. 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).

1. Start Outlook.
2. On the Files menu, click Info.
3. Click Account Settings, and then select Account Settings from the list.
4. Click to select the target POP3 account, and then click Change.
5. Click More Settings.
6. Click the Advanced tab.
7. Gradually increase the server time-out setting until the issue is resolved.
8. Please see the attachment

source: http://support.microsoft.com/kb/813514