Hi Claire,
The most common reason that an Email Send will remain in the Outbox without sending is when you set a "Send On Date & Time" in the Email Send record, but never actually click the Send button.
I see that you opened a support ticket for this issue, and I encourage anyone else who is seeing scheduled Email Sends stick in the Outbox when they are sure they clicked the Send button to do the same. A support ticket will allow our customer team to do a more detailed investigation of your specific issue.