Hello eskimodoc,
If you're getting the message "unable to verify account or password", that would mean either the username or password for the account was entered incorrectly. Here's the steps from Xfinity (Comcast) for setting up their email accounts in the Mail application on Mac:
1. Click on Mail and select Add Account.
2. Select Add Other Mail Account…
3. In the Add a Mail Account window, enter the following information:
• Full Name: Your Name
• Email Address: Your full Xfinity Email address (username@comcast.net)
• Password: Your Xfinity Email password
4. Hold down the Option key and click Next (Next will only appear while holding down the Option key).
5. In the Incoming Mail Server Info window, enter the following information:
• Account Type: Select IMAP
• Mail server: imap.comcast.net
• User Name: Your Xfinity Email username (left of the @ symbol in the email address)
• Password: Your Xfinity Email password
6. Click Next.
7. In the Outgoing Mail Server Info window, enter the following information:
• SMTP Server: smtp.comcast.net
• User Name: Your Xfinity Email username (left of the @ symbol in the email address)
• Password: Your Xfinity Email password
8. Click Create.
Configure Apple Mail 7 for Xfinity Email
*You'll specifically want to pay attention to the "user name" section in steps 5 and 7. Make sure that you are only entering the user name portion and not the entire email. For example if the email address is "Apple@comcast.net" you would only enter "Apple" in the username fields. If you type in the full email address, it will end up giving you the "unable to verify account or password" message when trying to finish setting it up.
If you're still unable to setup the account in the Mail application, you'll want to contact Xfinity (Comcast) for assistance.
Cheers!