Emails can get stuck in your outbox for several reasons, but most commonly it is because your message is carrying an attachment that is too large to transmit through Outlook. Outlook can only send attachments that are less than 8MB. If you need to send an attachment larger than that, I recommend YouSendIt. You can upload large files, and then they send them for you as well as provide a link where you can download the file.
If you have an email stuck in your Outlook outbox and you can't re-open it, try this:
1. Go to the FILE menu, then choose WORK OFFLINE
2. Switch to your Outbox
3. You can either right click on the message and DELETE it, or you can drag and drop the message into your DRAFTS folder where you can slim down the attachment and try again.
4. If you still receive an error message that states "Outlook has already started transmitting the message," then you will need to close and re-open Outlook. Sometimes Outlook doesn't exit quickly. If that happens, press CTRL + ALT + DEL and find outlook.exe in your task manager and click "End Process." Repeat steps 1-3 when you re-open Outlook.
I've had this problem several times, and this has worked everytime. I hope it works for you! Good Luck!
