A while back the Email Marketing Manual wrote Spammers v the spam filter where they reverse engineered today's content filters to help us understand why filters are so fussy and to help us know how to make emails that don't get blocked.
Paul Cunningham from capslockassassin.com, on April 24, 2009, wrote Anti-Spam Products Are More Than the Sum of Their Parts which was published on allspammedup.com.
This is another very good overview of receiving systems' reaction to our emails.
My office, like many offices, uses spamassassin as its primary filter as it is free and open source and is plugged into most of the big useful black lists.
I recently got an email that apparently was from me and too me (to avoid safelist filters), with this content:
This holiday season, shop online at our luxury watch store.
We carry the largest selection of:
and dozens of other famous brands.
With prices staring from just $99 - they will make the perfect and most affordable gift!
Browse our shop (was a link to some weird url)
And here is what spam assassin said:
Now I do understand that but it is my job and I am a bit of an email geek but hey, that's where the money is!
At the top it says what the spam score is (the higher number the more spammy) and it says that my office, like most, has a threshold of 5 before it will recommend to send it to junk.
Most of the spam warnings are telling us the 'mime' has urls that are black listed, this could be in the content, plain and/or html or it could be the from domain.
This is spam assassin aggragating it's own content database and multiple spam listings for you one place.
This is not the only filtering on the office system though, we do have a manual IP black list as do most mail servers.
Some hosted solutions will do more and some gateways will also employ filtering before it even gets to your mail server.
Don't forget that your own inbox will also have a self learning filter which changes its rules every time you mark an email as spam or unmark an email as spam. If you are lazy with the spam button and don't use the optout link on emails that you have asked for and hit spam instead, you could be mis-training your inbox filter and it could cause false positives and send good email to your junk filter. This is why senders, especially for transactional emails, will always ask you to add their from address to your address book and/or safelist.
To summerise, Anti-spam products are more than the sum of their parts because they take information from many other sources and make decisions for you but there is always more the one at work at the same time!