This actually represents a couple of very common points of confusion. One is very easy to clear up; the other, not so much.
First, the easy: the terms junk and spam are synonymous. Some email programs or services call it spam, some call it junk mail, but it’s the same thing in either case. What you’re seeing in Thunderbird is what happens when those worlds collide.
The confusion about training the spam or junk filters, however, is both important to understand and somewhat more complex. While I’ll use your AOL account as an example, this applies to all email services and all desktop email programs1 that have spam filters.
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Spam versus junk
So, if it’s one or the other, why do you have both “spam” and “junk” folders in Thunderbird?
Thunderbird uses the term “junk”, and its junk mail filter moves what it considers to be junk into a folder called “junk”.
AOL uses the term “spam”, and its spam filter moves what it considers to be spam into a folder called “spam”.
You probably have Thunderbird configured to access your AOL account using the IMAP protocol. (See What is IMAP? And how can it help me manage my email? if you’re uncertain what that is.) IMAP causes the folders on your AOL account to be downloaded into your email program…
…including the “spam” folder. So you end up with both:
- Thunderbird’s “junk” folder
- AOL’s “spam” folder
How local and remote spam folders interrelate
When email sent to you is received by AOL, it is analyzed by AOL’s spam filter, and email considered spam is moved to the spam folder.
When downloading email into Thunderbird, that email is analyzed by Thunderbird’s junk mail filter, and email that is considered junk is moved to the junk folder.
Knowing that “junk” and “spam” are two words for the same thing, you can now see you have two completely separate spam filters working for you:
- AOL’s, on the AOL server, when your email arrives and before you download your email
- Thunderbird’s, on your PC, when you download email
In theory, Thunderbird would mark as spam/junk anything that AOL might have missed.
It’s kind of an accidental belt-and-suspenders kind of situation.
What trains who?
The real confusion happens when you mark something as spam or junk in order to train the filters. Which filter are you training?
When you mark something “as junk” in Thunderbird, you are only training Thunderbird’s junk filter. The AOL spam filter never hears about it, and thus has no way to know.
In order to train the AOL spam filter, I believe you actually have to log in to AOL’s web mail and mark things as spam there. That will move the message to AOL’s spam folder, and should even cause it to appear in the spam folder when you view it in Thunderbird. But in this case, Thunderbird’s junk filter will not have been trained, because it has no way to know what you did online at AOL. That the mail appears in the “spam” folder means nothing, since to Thunderbird that word has no special meaning; what AOL calls “spam”, Thunderbird calls “junk”.
I don’t believe simply moving messages between the junk and spam folders trains anything.2 I think it’s the “mark as” action in either location that actually does the training, but only for the location in which you perform the action.
If you want to train both AOL (the service) and Thunderbird (the desktop email program), you have to mark spam or junk in both places.
Other services and programs
This confusion happens all the time, and applies to all email services that filter spam online, as well as all desktop email programs that filter for spam.
That you happened to have both a “junk” and a “spam” folder just made it more obvious.
When your email service and your email program agree, and both use the term “spam”, for example, you may have two separate spam folders – one managed by the email service’s on-line spam filter, and the other managed by your desktop email program.
Or you may have a single spam folder into which both place the messages they identify as spam.
But one thing remains constant: when you “mark as spam” (or junk), that trains only the online service (if you’re doing it in a web interface) or your desktop email program (if you’re doing it on your PC). I know of no way to tell them both at the same time.