The problem with cwmx.com is that it doesn't work when you're using WiFi.
A better solution is to use a different mail port, one that Comcast will accept. Comcast blocks the standard port 25 when you're sending mail from outside their system. I found that port 465 works for my Comcast account. (Don't know if it matters, but I'm located in Massachusetts. Port 587 is one that people use with other providers, but it didn't work for me.)
To set it up, go to Settings/Mail on your iPhone, select your account, and scroll down to "Outgoing Mail Server (SMTP)". Then enter these settings:
Host Name:
smtp.comcast.net:465
User Name:
Your Comcast mail user name (without the @comcast.net)
Password:
Your Comcast mail password
Under the advanced tab:
Outgoing Uses SSL is ON
Authentication is set to Password