If I use one of my comcast accounts it works fine.
If I use the second account it get a "Username or password incorrect on mail.comcast.net" message.
I have verified the account is valid by logging in to www.comcast.net, m.comcast.net and smartzone.comcast.net. I can log in fine and there are emails there.
I've tried it on both phones and get the same result.
I can set either phone to the first account and it works fine.
I've tried power cycling the phone, turning the account off and on, verified the username and password several times, tried the incoming server as mail.comcast.net and pop3.comcast.net, tried ssl on port 995 and no ssl.
I've tried setting the phone to the first account, verifying that it works, and then changing the username and password to the second account without changing the other settings.
If I ignore the errors and set up the account anyway and then try SENDING an email, it sends fine.
Gets to the recipient intact with the correct senders address.
If I reply to it, it goes to the correct account and I can read it online.
If you have successfully set up an account on one phone, I guess we can assume you know how to configure an iPhone correctly for a Comcast email account
What I would do on the problematic phone is delete the account the account and start over. Ensure all info is entered correctly. Outgoing info: smtp.comcast.net; Port 587; SSL ON; Password Authentication. Incoming info: mail.comcast.net; Port 995; SSL ON; Password Authentication. Watch that iPhone doesn't auto-capitalize any characters when entering your password.
How about if you try resetting the password on the problematic account in Account Management (click Manage My Account under Customers in the blue bar above, login again, then click Users & Settings). Go there and set a new password on the account, reconfigure the iPhone, then try and send yourself a message. We've seen users have similar issues occasionally with desktop-based email clients. It's worth a try.