I had exactly the same problem with the same (bogus) email address, firstname.lastname@example.org.
Just started yesterday.
The answer above is correct. In "Settings" on the web email site I found that auto forwarding had been enabled and that email address had appeared as the place to autoforward to. No idea how that happened. I deleted the address and unchecked the enable autoforwarding box.
Something strange going on with the xfinity email site.
... View more