Lately I was unable to send emails using the gmail app on both my Android phone and Xoom tablet. They happened at different times but happened after installing a rom or updating to a newer software version. I presume this problem is pretty widespread.

I tried re-syncing as well as deleting my gmail account and re-adding. Nothing worked. Finally I tried going into settings -> manage applications -> gmail. I selected clear data and voila, it worked!

Note: once you clear data any emails in your outbox that were not sent will get deleted. You should copy the text or try saving the email before clearing data.