Desktop, laptop, phone or tablet? IOS, Android, Mac, or Windows? What email program, app, or webpage are you using?
What happens when you send a test email to the address? Any error messages? Does the test email show up on https://connect.xfinity.com/?
The more details you can provide, the better.
If an employee doesn't reply to your message here, Comcast Security Assurance might be able to help. You can reach them at 1-888-565-4329 (from https://internetsecurity.xfinity.com/help/report-abuse/). Note that this is NOT one of the general customer service numbers.
The first-line reps there are sometimes not very helpful, so you may need to be persistent and keep at them until you get a satisfactory answer.