They do show up in my sent folder

That means the message has been sent, i.e. Thunderbird delivered it to your email provider's SMTP server. Whatever happens next is beyond your (and Thunderbird's) control.

Messages can go lost any where in between your providers server, the recipients provider server, and the recipients computer.

The only thing you can do is to check with your email provider whether the do deliver your messages to Yahoo. For everything else you'll have the recipient to check what's going on.

Thanks for the response. When I was using Windows Live as my mail client I never had this problem. As soon as I switched to Thunderbird the problem started. I have kept the email provider constant so it doesn't seem likely that it is them. I have used the same email provider for over 15 years and never had this problem. Does it seem just a coincidence that this began with my beginning to use Thunderbird? Also, have worked with both of my friends to make sure I am entered correctly in their address book, etc. They are as puzzled as I am. I have sent emails to them for over 10 years and never this problem until Thunderbird.

Does any of the recipients with the problem use Thunderbird?

No they don't.

That's the only thing I can think of where Thunderbird is involved. https://support.mozilla.org/en-US/questions/1119474

But it doesn't exactly apply to your situation.

Thanks for your time and help!