I have recently had this problem, deleted account and added them back in, could recieve emails but not send.
Following the advise from wjosten above I went to the account under settings, tapped on the email account for mail, at the bottom of the next window SMTP for the outgoing server I had 'Other SMTP Servers' where I changed the port numbers to those that were recommend by my mail provider.
My mail has recently been change (BT) to an Outlook account, the BT site gave very clear information for the teehnical 'stuff' required, even a iPhone setting page, very helpful.
What provider are you using? Have you checked their site for any specific guides for setting up email?
I did find that deleting accounts off my iPhone didn't actuall delete them, which retained conflicting information.
I even 'turned it off and back on again' LOL
Message was edited by: time18 JUST NOTICED THE DATE ON THE ORIGINAL POST, D'OH!!!! Might help someone looking for advice....