What is SPAM?
Spam is not unwanted mail, it is unsolicited commercial mail. Wikipedia simply calls it unsolicited bulk email but it is important to draw a line. Unwanted email is not SPAM.
It is very important that everyone understand this or they will spend a lot of time complaining about their SPAM filter not working correctly because it does not remove some mail or other that they do not want but which is not SPAM. It is also important that you understand the basics of how SPAM is identified by a computer program.
How do SPAM filters work?
There are two basic types of SPAM filtering, there is what is called Whitelisting / Blacklisting and then there is Bayesian Filtering (named after Thomas Bayes).
Bayesian Filtering uses the mathematics created by Bayes to apply probability to the content of an email message. The true strength of this mathematical probability is that it is a learning process. The success or failure of previous analysis is stored and included in the next round of decision making. The net result is that over time the filter 'learns' your preferences. This is the type of filter Thunderbird has.
White Listing is providing a list of known good senders who do not send spam. Those in your Thunderbird address book are automatically included in a white-list that Thunderbird applies before it gets to it's Bayesian filter. Some providers take this to the extreme, and offer to exclude anyone not in your address book. This is just lunacy. It means that you don't get mail from anyone not in your address book which includes things like emails you subscribe to, mailing lists you join etc. (I am on a number of mailing lists that are not in my address book. I read what they send, but I don't need to reply so they are not in the address book, Ebay mail me saved searches daily. I don't reply, but I do want to see them)
Black listing is the process used by many SPAM filters, and it can be and often is the bane of peoples lives.
The idea behind a Black-list is that addresses on the list are known bad senders and should be excluded as a matter of course. One US based ISP takes blacklist entries from their customers, if two customers say a mail is SPAM then it is SPAM as far as they are concerned for everyone of their subscribers. This has caused all sorts of trouble for others using the ISP who suddenly can't get mail from their yahoo groups because a couple of clowns flagged mail on the list as SPAM, rather than actually unsubscribing from the mailing list.
Then there is the ever contentious "how do I block this person" Some months ago I posted on this topic, with the simple instructions on how to do the impossible. You as a mail user can't block someone from sending you mail. Sure Outlook Express and other mail programs have a handy little button that says something like 'block user' but it does nothing of the sort. The mail has already been delivered to your account. Your mail provider accepted delivery on your behalf. All you can do is filter the mail as I described so you don't see it. This is basically all your 'block' buttons in other programs do. They build a list of those you don't want to see mail from and junk it. A filter in Thunderbird works just as well.