Hi Jerry,
To check and manage your From: address go to the left side navigation in Thunderbird, then right click on the account and select settings.
The From: address, at least on Windows 7, should be the E-Mail Address field that I highlighted.
You also have the option to Manage Identities in case you want to be able to send From: different email addresses.
In this case it sounds like you may be required to send From: the same address that you are authenticating as, and that you should double check your settings for typos too.