I assume you have already changed your password using the Xfinity/Comcast website. If that has not been done yet, then the password needs changed there first.
The outgoing email setting is called the SMTP server, so on your phone, go to Settings > Mail > Accounts > [name of account] > SMTP. Where it says Primary Server, you will see smtp.comcast.net. Tap that and enter your new password in the Password field.
For the Mac, go to Mail > Settings > [name of account] > Server Settings. Under Outgoing Mail Server (SMTP), there is an Account option with a pull down window, select it and choose "Edit SMTP Server List". Enter your new password in the Password field and save that. For good measure, I also put the new password in the Password field on the screen that it returned to after changing it in the Server list.
The incoming Mail Server password is the same as the Outgoing Mail Server, so the Incoming Mail Server should also be your new password that you already created with Xfinity/Comcast.