i had this problem. i fan a fix permissions through rom manager. that didn't fix it (although it's a good idea to do it once in awhile) I went to settings > applications > manage applications > Gmail and I clicked Clear Data. started gmail back up, it had to re-log into gmail which only took a couple seconds. the email I was trying to send was no longer trying to send. I re-typed it, hit send and voila! it sent.
hope this helps.