server setting should be port 993, SSL/TLS, normal password.

Thanks for your reply David. I have the server setting as imap.postoffice.net/ port 993 / SSL/TLS and the outgoing server as smpt.postoffice.net / port 465 / SSL/TLS. Frustrating.

outgoing should be port 587 and STARTTLS . You can clear/reset passwords at settings>privacy&security. click savedpasswords and then showpasswords and then highlight both rows for the account and delete them. That should clear out whatever is there so you can then be prompted for password.

Thanks again for your help. I changed the port to 587 and STARTTLS. When I click on show passwords a window opens that wants to verify my identity with my user name and password. I tried what I thought they are, but it says they are incorrect. How do I retrieve them?

usa.net uses same id and password as website. Login at website and those should be same as for PC. I hope you saw my typo of stmp.postoffice.net, which should be smtp.postoffice.net

I used smtp. When I enter the same user and password that I use to open my usa.net email account, the Mozilla Thunderbird security window still says "The username or password is incorrect". What am I doing wrong? Have I somehow reset the username and/or password with Thunderbird while I have been trying on my own to get this to work (before I asked for your help)?

I think that is a prompt from Windows, wanting your signin password. NOT your usa.net password.

You were right. I entered the Windows password and username and am able to see the password for my usa.net username. This is in a window called "Saved logins", correct? In an earlier response, you said once I was able to show passwords that I was to then to "highlight both rows and delete them". There is only one row for the usa.net username, the one that is showing the password. Is that the row I should delete? Do I just click on remove with that row highlighted?

Thanks for your patience.

Yes, you will then be prompted for your email password. I am pleased to assist.