All email accounts have some way to identify you, the user, to make sure you're entitled to use the account. Almost universally, this means an email account has a username and a password. The username is usually the email address, or part of it.

You need to know the password for your email account, and then store it in Thunderbird.

In the working machine, you can see stored passwords here:

Tools|Options|Security|Passwords→Saved Passwords

I was using the correct PW, but TB said it wasn't "valid." I have the very same settings on my desktop and TB does not ask me for a PW.

 On the NB I deleted all the PWs, but TB still asks me for a PW.
 Any suggestions?

Vumbo said

I was using the correct PW, but TB said it wasn't "valid." I have the very same settings on my desktop and TB does not ask me for a PW. On the NB I deleted all the PWs, but TB still asks me for a PW. Any suggestions?

Mrssages about invalid login or password often indicate some other problem that prevents Thunderbird from successfully connecting to the server. Anti-virus and firewall software are common causes of this.

I have the same antivirus/spam and firewall on both my DT and NB computers.

I am forced to enter my server PW - I just noticed it appears in my replies at the top of my reply emails folling my email address. Perhaps others as well. This is bad!