Edit - apologies - I mstakenly said Ipad when it was your Iphone - however they both use the same operating system. The Iphone's smaller screen makes some things appear different, but the underlying systems are exactly the same.
Lets use Thunderbird to see what additional information we can gather as to the problem. If you can receive but not send I'm guessing it could be a 525 VM 304 error - However rather than make assumptions I'd rather see what Thunderbird shows.
Only use Helpful answer if your problems been solved.