emails sent from thunderbird to aol addresses are not being received by the aol recipient. how do you fix this?

There is no such thing as a 'Thunderbird account' or 'thunderbird email address'. You've set up an account with an email provider, e.g. Gmail or Yahoo. You told Thunderbird to retrieve/send mail for that account from/to your email provider's server.

When you see a copy of the sent message in your Sent folder, the message has been sent, and the problem is not with your Thunderbird installation.