Why do I have this "Email Setting" issue in my account setting?

In my account setting, I have this “Email Setting” issue. It says “We were
unable to deliver email to your '????@gmail.com'. The last date we
attempted to send you an email is 12/15/20, 3:24 PM.” It adds that: “Email
address does not exist.” My email address exists and I checked it myself and
I can send and deliver email to it. This causes another issue for me; it does
not allow me to check my messages in cargurus and when I try to go to my
message box it shows this error: “Please verify you have logged in with a
valid email and try again.” I called the customer service, sure enough I was
not able to talk to a real person and the system asked me to leave a
message. I did so and I am exploring other venues to resolve this issue. Any
help would be greatly appreciated.