Xfinity sends service-related emails to wrong email address

When Xfinity sends service-related emails, I get them at the email address associated with my Xfinity account and I also get them at an email address I've never had on my Xfinity account but which I used to sign up on an Xfinity-affiliated website. (I changed the email on that website to match the email on my Xfinity account years ago.) Since the emails are service-related, there's no option to unsubscribe in the email. I've spoken with Xfinity about this several times and they always claim this second email address is no where in their system. The emails come from the exact same email address as the ones sent to the email address on my Xfinity account though, on the same day, and they have the same information, which at times has included the serial numbers of my set-top boxes. So I know they're legitimate. I've verified the email account I use with Xfinity isn't set up to forward any email. I talked with the Xfinity-affiliated website and they said they would never send Xfinity service-related emails because they don't have anything to do with my Xfinity account, and they also no longer have record of the email address I used when I signed up.

Is anyone else having a problem like this? If there's a solution other than blocking Xfinity emails on this email account, I'd love to hear it. I've tried working with Xfinity on it enough times that I'm not going to waste more of my time on it with them.