Is this happening with Comcast's webmail ("...connect.xfinity.com..."), or with an email program or app? When does it happen, loading emails, sending them, logging in, or ???. What is the complete error message (if it includes an email address, remove that part before posting)? If webmail, what browser are you using? What kind of device (desktop, laptop, tablet, phone? Mac, Windows, Android, IOS?). What device model? what OS version?
The more details you can provide, the better.