Sometime, your iphone fails to send email messages. Then it tries to send the email messages repeatedly, until your battery gets drained.
1) Go to settings.
* Enable "Airplane Mode".
* In Mail etc, disable the email accounts. Set to "Off".
2) Now go to the Email account where you are having outgoing messages issue.
3) In the OutBox for the email account, go through the stuck emails and delete them.
4) Go to settings and disable "airplane" mode and re-enable the email accounts.
This should work.