Have you tried just quitting mail and the restart the iPad. Double tap the home button - bring up the task bar - tap and hold down on the mail app icon until it wiggles - then tap the minus sign to close the app. Actually - you might want to try closing all the apps in the task bar. Then restart your iPad - and try again.
Have you tried turning on other SMTP servers to send mail?
http://support.apple.com/kb/TS3234
Finally - and I did this one time - I was having a similar problem so I did delete the mail account. I went through fits trying to add the account back onto the iPad. Just would not work. I was baffled - this mail account worked flawlessly for months and now I could not set it up on my iPad again. I decided to restore my iPad from the backup and the mail account began working flawlessly again.
The only caveat to this is that I restored the very next day after the account would no longer let me send mail so I had a backup from the day before when the mail account last functioned properly. For all I know it was even merely coincidence that mail started sending again from that account - but I was able to restore the email account and all of the settings from that backup.
Don't know if that would work for you or when you last backed up ....