A customer had switched their POP3 email and internet service to a new ISP. Which required them to change their POP3 and SMTP settings on a couple of users' iPhones. The users were able to receive but not send email.  When initially setting up POP3 on an iPhone it requires adding the ISP's SMTP settings and then once it is configured changing it to AT&T’s which is there by default (cwmx.com). This change only has to be done if the SMTP server for the ISP is using port 25. This is because AT&T blocks port 25 on their data network for wireless and internet customers. Sometimes you can get them to unblock it for your account but that is rare.

One of the iPhone users also had a laptop that he only uses with an AT&T Express Card on the GPRS network. He could receive but not send email. Since he was accessing his POP3 account via the AT&T data network port 25 was blocked. I entered “cwmx.com” for the  SMTP server just like for the iPhone and was then able to send.