That is a comcast email server issue to resolve. Thunderbird is a email retriever and send through comcast by itself it does nothing with a email server to connect to. Also this is Browser help forum there is a Thunderbird forum that you should post Thunderbird issue to as well.

This post appears to be in the Help/Support Forums/Thunderbird/. Is there another place I should be asking this question? I don't understand markwarner22's reply. My problem is: 100s of spam emails come into my thunderbird account from my Comcast.net email address that never appear in the Xfinity webmail page. When I go to Xfinity's webmail webpage, none of these spam emails appear anywhere. Neither in its Inbox or Spam folder. They're just not there. But somehow, when Thunderbird "retrieves" email from that account, it "retrieves" all these invisible spam emails from somewhere. It appears Thunderbird gets them before Xfinity (Comcast) filters them out. BTW, I have no filters set up on the Xfinity webpage. So however Xfinity gets rid of the spam, it does it on its own.