Please understand that Thunderbird can never, itself, be blacklisted, since it is running on your account. It's like sitting in the back seat of a car that is stopped for speeding. It's the person driving that's ticketed, not the passenger in the back seat. So, you need to delve deeper into this. It may be that you are blacklisted for content you send, or blacklisted for your account sharing an IP address with someone else who is.

David. Just checking all angles. Seems everyone is blaming everyone else. It is my IP Address that keeps getting blocked. And I figured Thunderbird would not work if the IP Address is blocked. So been checking with my VPN service as well as Comcast. Waiting for a reply from Comcast but I have a hunch I will need a dedicated IP Address which I hope will solve the problem

Thunderbird is 'riding on' your IP address. It sends messages and the messages are blocked because they are deemed to be coming from an unsafe IP address. It is not an email client's presence that is blocked, it is the sending of messages that is blocked.

That is what I figured. I believe it is Comcast doing it probably because too many people are using the same IP Address. Thanks for your responses. Will look into a dedicated IP Address.