To overcome this issue, go to Settings > Account. Tap on the account you use to send emails and then activate the SYNC option for email. Switch to the email application and go to Outbox folder. Connect to the Internet and then press the menu button. You will have the possibility to use the Refresh button to force send the emails. Next time you'll send an email, the process should be automatic.
Additionally, go to Settings > Application Manager > All > Gmail/Email > Force Stop and Clear cache.