I am not sure if this issue is going to help you but lets give it a try.
I know that where im from, which is the Netherlands, I am not able to sent e-mails from my iPhone or any other mobile device by using 3G or a random WIFI acces point. The provider only allowes you to sent e-mails when you are connected to a network of there own. Which means if you have provider "101" and your email settings are set up for this provider in your iPhone but your using the network of a friend of yours which is provider "202" you cannot sent emails from his network. Though you can receive them you may not sent.
As I said, I am not aware if this issue is the same for you totaly depents on the country and provider your using.