My Comcast email stopped loading on my Mac Mail program. Comcast advised changing the password. I did and now, while the mail will load on the Xfinity site it still won't on my Mac Mail. Spent six hours on the phone with Comcast today!

While taking to the ISP for 6 hours, was it every mentioned that is made Required the User to have an Application Specific Password

Refer to link below on trouble shooting this issue with your e-mail account with the ISP.

If Mail on your Mac keeps asking for your password - Apple Support

Excerpt from above link

Your email provider may reject your password for several reasons, including:

• You're not using your current email password. If you updated the password on another device, make sure that you're using the same password on your Mac.
• Your email provider needs you to update or reset your password for security reasons. For example, if you're using Google’s two-step verification, your Gmail account in Mail requires the appropriate app-specific password.
• Your email provider deactivated or suspended your account. Contact your email provider for help.