This actually happened to me yesterday! Not sure why. I tried to reply to an email and it stayed in my inbox and just said "Sending" in red. After awhile of not sending, I just deleted it and sent from my regular computer.
I just thought of something else. My email is also not syncing right over the past few days. I am getting the emails right away, no problem, but when I delete from my phone, they still show on my computer.
Interesting, the email on my X is not syncing either. I have to manually refresh. Just tried to forward an email and it says 'email can be sent over wifi only'. I usually run 3G since only wifi usage is at home. How do I switch back to 3G for receipt of emails?