This happened to me today, I replied to a mail with large attachments, and the mail got stuck in sending state, and didn't send. After reading many forums, the following fix worked for me:
- Open Outbox
- Select original offending message
- Scroll to end and select REPLY, and then SAVE Draft.
- Close Outbox and open Drafts
- Open email and you can then access the unsent message, copy it or delete/discard it.