Junk does what, exactly?

[Macs R We]

I can't find any good reference as to exactly what effect the "Junk"  
button has in Apple Mail.

Sure, it moves a message to the junk mailbox, that much is clear --  
but what gets "learned" in terms of future mail?  Does it become more  
wary about messages with similar contents?  Does it give the sender a  
demerit?  Neither, both, something else?  If so, how many demerits  
put all of that sender's messages in the Junk bin?

And how can you see what Mail's plans are for future messages?   
"Learning" based rules don't appear in the Rules panel.  For example,  
don't expect to find a rule saying, "It the contents contain Viagra,  
and the contents contain shipping, it's junk mail," unless you put it  
there yourself.

In fact, a number of online references seem to imply that once you  
take Mail out of "learning" mode, the Junk button no longer causes  
anything to be learned.  I never thought that was the case, but I've  
just realized that I can't point to any authoritative source that  
says otherwise.

Here's my motivation: today, a new client complained to me that once  
upon a time she "junked" a single chain-letter message from a friend,  
and now she regrets it, because all that friend's messages are going  
into the Junk mailbox.  (That struck me as odd, because my experience  
has been that you have to hammer a sender multiple times before he is  
effectively blacklisted, if ever -- usually I get tired and just  
write a rule.)  She couldn't exhibit any proof, since her Junk  
mailbox was empty.  She wanted me to do something specific to undo  
what she had done.  As I remarked previously, it's not like there's  
some file I can go to and remove the line that says "Junk all mail  
from Hermione."

So I told her next time a message of Hermione's got junked, she  
should go into the Junk mailbox and un-Junk it -- on the theory that  
whatever weak censure a single Junk button may have caused, a single  
un-Junk button should correct.  Then she said, "I can't tell when  
there are things in there, and they don't stay in there long."  Now  
first off, I know for a fact that the unread message count shows up  
right next to the Junk mailbox.  Secondly, I can find no preference  
setting that would automatically empty the Junk mailbox, so I don't  
think that even happens.  So at this point, we're touring Whine  
Country, which is a different and non-technical problem...

Anyway, it struck me after this encounter that Mail's claims of  
"learning" are vague and unsatisfying, in terms of exactly WHAT it  
thinks it is learning, how often it has to "learn" something before  
it "sticks," and what you can do to modify what it has learned.  I  
could find nothing in a Google search other than the usual vague  
promises that "Mail learns stuff when you hit the button."

A colleague at AMUG suggested this list as a place that the actual  
developers and other gurus might have pointers to more specific  
references.  I appreciate whatever you can offer.

-- 
   Macs R We -- Personal Macintosh Service and Support
     in the Wickenburg and far Northwest Valley Areas.
                             http://macsrwe.com