if Mail goes to your sent folder without error, Thunderbird has received a "sent ok" from the outgoing server. Your mail is not arriving, time to talk to your service provider. They probably have spam rules and are quarantining your mail somewhere or are simply dropping it as SPAM. There is no guarantee of delivery or of notification of failure in the email protocols and the larger ISPs in the US especially make significant use of that absence in their cavalier spam policies.

I should have added that I use ATT internet and Norton Security Suite from Comcast.

Since my original post, I sent a test message to my own address as well as my SO's. She got hers, I did not get mine. Would ATT quarantine just one?

ATT / Yahoo do not deliver mail to yourself They treat it as spam.

Thanks Matt, but in the past I was always able to send myself mail and I just sent a test email to my self a few minutes ago and it showed right up. Perhaps ATT had a temporary glitch that is now solved.