Here are a collection of tips to help you solve problems with sending or receiving email:
I can receive email but can't send any. What do I do?
When you are connected to a Wi-Fi network, you may find that you can receive email successfully but that you get an error message when attempting to send email. This typically happens because of anti-spam actions taken by the ISP for your currently connected Wi-Fi network.
To work around these restrictions, so as to send legitimate email, try one or more of the following:
Turn off Wi-Fi: This forces the
iPhone to use EDGE or 3G. You may also need to go to Settings >
Mail > name of mail account and, assuming
you are using AT&T, set the outgoing (SMTP) mail server to
cwmx.com. No user name or password ...