Spectrum have been using a rather buggy in house mail server software for a number of years, so it really does not work like it should. Going on from there, I am guessing you also get that account delivered to your phone and that the server fails to actually declare the folder to be used for spam. So you have three, one from each of the sources.

Thunderbird defaults to use Junk. So I think we can start by eliminating that. This is just a guess, based on what I see in this topic https://support.mozilla.org/et/questions/1252481 but it appears spectrum use junk mail.

So go to account settings, look in junk settings and point the junk folder in use to the junk mail folder. Then drag any mail in the existing junk folder to junk mail and delete the junk folder completely. I would suggest you just empty the spam folder into junk mail as well and then delete it. Just see if it comes back. That would indicate another device with another mail client is using that folder if it does.

Now you your white listing issue. look in the spam mail folder and check the junk status (the flame icon) in the list. If it is orange, Thunderbird put the mail there. If it is just an outline, spectrum put the mail there.

I do not know how Spectrum whitelists, but it might be you need to have contacts on their web mail to stop them treating mail from them as spam.

Hi, Matt, and thanks for your response. I've done as you suggested, but when I tried to delete the excess folders, I get a Thunderbird message that says "The current command did not succeed. The mail server for account [my Spectrum email] responded: the mailbox could not be renamed."

Interestingly, I got the same message when I tried to delete "Spam," but it did delete it anyway. However, it popped up again when I closed and re-opened Thunderbird. However, both Junk and Junk Mail folders are still there. There isn't even an option to delete the Junk folder on a right-click.

As you suspected, the flame icon on the messages that route to junk over and over is just an outline, so Spectrum would seem to be the problem. I rarely access my email on my phone, but when I checked just now, there's no place where my address book shows, so I suspect Spectrum is just referencing my address book from its own server and if I wanted to add or delete anything, I'd have to do it on the desktop or my tablet. This seems peculiar to me, but I'll call Spectrum tomorrow and see what they have to say.

Thanks for trying, once again, to help me sort out my ongoing issues with Thunderbird. Happy New Year!