In iOS, you need to set the password in two places------for the incoming server, and then again for the outgoing SMTP server. Did you do that? If not, to change it for the SMTP server;
In iOS go to "Settings" on the home screen, then scroll down to"Accounts & Passwords", click on the account, and then on the next screen tap the "account" again. Scroll down to "SMTP" and tap that. Then tap "smtp.comcast.net" again. On the next screen, tap "smtp.comcast.net", and you will see the password. Change it and then tap "Done" in the upper right-hand corner of the screen.
If that doesn't do it, delete the account from the Mail app and re-create it again from scratch. Let the phone auto-configure it for you after you put in the user ID and password.