how do you know you can't? An error message I assume, if so what is it?

I've never gotten an error message and the emails sent from thunderbird show up in my Sent file as usual. However, I know the messages don't come through because I sometimes send emails to my yahoo account and I've tried it with my husband's yahoo account as well.

So your issue is the messages are not being delivered to yahoo accounts, not that Thunderbird is not sending them. Two very different situations.

Yahoo is actually a fairly terrible mail provider these days. But my guess it for reasons only comprehensible to yahoo the are either placing the mails in the Bulk/spam folder, or simple deleting them because they don't like them for some reasons. What they do at yahoo in the name of spam mitigation is neither comprehensible or advertised by Yahoo. Information appear on an anecdotal basis.

These folk appear to have given the yahoo not getting mail thing some serious thought. https://www.lifewire.com/fix-yahoo-mail-not-receiving-emails-4782933