The IP address allocated to you by Comcast on that device (I assume you are using Comcast as they don't allow you to send mail unless you are) has been blacklisted by (you guessed in) Comcast. I suggest you ask them to explain. (If they can)

Hard to know where the problem arises. When i try to access my Comcast email on Thunderbird I often get the message of the temporarily blacklisted IP address. Incoming messages are then also blocked in the Windows email box that shows up when you click on the Windows icon in Windows 10. OTOH, even when this happens I can log in to Xfinity on a browser, either Edge or Firefox, and have full access to incoming mail. Can someone explain this?

I'm having the same issue right now. I had also had this issue previously (only through Thunderbird, access from this same computer to the web mail had no issue). The last time, about 2 or 3 days later my mail started flowing again as if nothing happened. It seems to me that the issue is the connection between Thunderbird and Comcast but I can't tell. I'll update when Thunderbird can connect again.