Cancelled earthlink account

Please explain what you did in more detail.

C;licked on Mozilla Thunderbird. Clicked on the mailbox for verizon.net. Wrote an e-mail; hit send; box came up with message that login to server smtpauth.earthlink.net failed; 2nd box came up with message to verify outgoing server setting. Complicating factor: I know very little about computers so not sure how one verifies or changes an outgoing server. Other complicating factor: not sure how Mozilla Thunderbird works as I don't think it was on the computer before I moved to my current address. Please let me know if more information is needed.

When comes the part where you canceled your earthlink account, and how is that related to the problem?

box came up with message that login to server smtpauth.earthlink.net failed; 2nd box came up with message to verify outgoing server setting.

Assuming that is your actual problem, see https://support.mozilla.org/kb/cannot-send-messages

not sure how one verifies or changes an outgoing server.

You talk to your email provider and ask about the correct settings. Then compare that with your account settings in Thunderbird.

not sure how Mozilla Thunderbird works as I don't think it was on the computer before I moved to my current address.

Not sure what's your point here. Thunderbird certainly didn't install itself out of the blue. Someone did it, and set up the account.