The quick answer is that Thunderbird uses "Junk", and whenever you meet "Spam" you can be sure it came from somewhere outside Thunderbird.

Thunderbird has built in Junk Controls and these work by you training them by marking messages as Junk or Not Junk. It counts the appearances of words and correlates their frequencies with the Junk or Not Junk classification you apply. It's not perfect, and many "spammy" messages contain invisible and innocent-looking text culled from various sources, thereby diluting the spamminess of the message.

If you see the word "Spam" then something else is at work. This can be your email provider or your own anti-spam software. It can work in two ways: move the messages into a designated folder (usually named "Spam", though some use one called "Junk") or label the messages, usually by inserting warning text into the subject line.

If your email provider actually moves messages into a Spam folder, you will only see that Spam folder, and the stuff inside it, if you're using an IMAP-connected account. Responsible providers would then treat POP and IMAP accounts differently.

If you use POP, you won't be able to see the Spam folder through your email client, so they might allow the spammy messages to be placed in your Inbox, but labelled. Some spam tools also add headers which an email client can detect and use. SpamAssassin and SpamPal are for instance supported and recognized by Thunderbird's Junk Controls.

If you're using IMAP then you'll be able to see the Spam folder, so it's usually more helpful for them to put the spam in there for you.

Thunderbird's Junk Controls will make mistakes, so you do need to review what is marked as Junk and what appears in your Junk folder.

In my experience, other people's Spam filters also make mistakes. Gmail's, in particular, seems to think that all messages from this forum are Spam, and it's tedious having to unmark them. There doesn't appear to be any effective way to whitelist trusted senders. Yahoo are even worse. While Gmail say that if you put a sender into your address book, their messages won't be seen as spam, I find that doesn't work. Yahoo don't even offer the option of whitelisting.

I can't make up my mind if moving messages between Spam and Junk folders is useful. It could be argued that if gmail's spam filter is working well, and you train your Junk Controls on the contents of the spam folder, you're training Junk Controls on proven spam (assuming you've checked it over first!) Likewise, feeding your Junk into a Spam folder might help educate the anti-spam system - that is, if it takes any notice of what you add to that folder!

My problem is similar. Due to a move, I recently changed my Email provider from Comcast (Pop server) to Gmail (IMAP server). Both accounts are still active, since Comcast continues to accept messages to my old address even though I haven't paid for the service for over 6 months. With Gmail, I have both a Junk and a Spam folder. Even though I THINK I have set both GMail and Thunderbird NOT to due adaptive filtering, stuff still shows up in my GMAIL Spam folder. My problem is that there is no indicating that there are unread messages in the Spam folder unless I explicitly click on the folder. Until I figured out this was happening, I missed important valid notices that were NOT spam or junk.

How do I get Thunderbird to alert me to the fact that there are messages in my Gmail Spam folder that may not actually be spam?

Thaks for any advice you can offer

Try this:

Right click the Spam folder, select Properties. In the Properties panel, tick the check box to include it when getting new messages.

A trick I use a lot is to go to View|Folders|Unread to see unread messages. But I'm not sure this will help you if it hasn't yet populated the Spam folder.

Sounds like this is EXACTLY the clue I needed. Ever since I discovered the problem existed, as a workaround to avoid missing important messages, I've tried to explicitly click on the Spam folder every day, but it's hard to remember to do that consistently. This is a GREAT relief.

And "View | Folders | Unread" is a very helpful feature I was unaware of. It's very convenient to see a summary of all unread messages in all my E-Mail Inboxes. (I agree that it probably won't work on folder contents that haven't been downloaded yet,)

Of course I'm still curious as to why ANYTHING is ending up my Spam folder, when I THINK I've set Google's parameters to stop doing all the oh-so-clever filtering of messages. But now that I've got a practical solution to the problem, that's a VERY low-priority curiosity.

Thanks very much for your prompt and helpful reply. I really appreciate it.