How do you expect Thunderbird to know all the valid email addresses out there? It does mark addresses that you enter in an address field with red text if they are not in your address book to give you a chance to double check that. If you re asking about getting a non-delivery notice for a message that you have any, that is a function of email servers, not Thunderbird

I was filling out a form on a U.S. government website once that told me an email address was wrong. Apparently the site connected to a service that attempted to contact the mail server to check whether the server would access an email to the address I gave. I've never noticed that anywhere else.

Is that the kind of thing you're looking for, to check whether an address is deliverable before sending? You could investigate available services, but I'm not sure you can plug them into Thunderbird: https://www.google.com/search?q=email+address+verification