I have to agree with txbakers.
Myself... I don't use SpamAssassin but I block emails a more technical way via my mailserver using "regex" coding to specify IP numbers to block. Today I have nearly all of Asia (APNIC ip numbers) blocked as well as many areas in Europe are blocked by IP number (first received line ... at my mailserver ... can not be faked ... in the email headers.
I agree that advertising via email is dying mostly becuase most is unsolicited, and most of it is foreign source for emails (or from ISP's that don't care about spammers) and most contains fraudulent links in the body. Ever get one of the PayP... or Cha.. Bank emails that look real (view its "raw" code) to see any true links in the body.... those might look like they're from PP or some place but they're usually not and if you click a link in the body and give them your login/password or card number or whatever they ask for you would be giving your info to someone in a foreign country (China, Taiwan, just to name a few).
Emails to your users. Did you mention originally to them that would be a condition of their usage, or is it done with permission specifically from them?
Either way, alike as txbakers said.... the spam filters must be set to very strict to block out the bad spam. The spammers keep trying to get past them so they use less garbage in their spam mail to be past the filters, etc, thus the filters get set to more strict.
As a result too of more strict filtering, often some Non-spam email gets caught and does not get to the user. People with SpamAssassin often do not check their spam box either. And in the case of my "hardnosed" filtering via my mail server by IP number, any spam that my mailserver blocks does not get to me, nor does it go into a spam folder, and it does not get replied to (not even a 550 invalid name, or any other "refused" code)... the garbage coming to my emails simply go to garbage never never land.
And with some of the existing laws against spam (California has a law, I believe Washington state, and other states etc) really there is not much done against spam, at least with regard to support to individual recipients who get spam.
The good part is.... in the eyes of uncle sam at least... is that it is cutting down on real email usage. Originally when the internet came about the "net" was viewed as a "good thing".... means of education, means of economy, and means of communication. People soon realized that they could "reach the world" with an email or with a web page, either of which might contain info about wrong doing by a company or gov't. Both companies and gov't soon realized this potential and now it's a pain in the you know where for them that the "little guy" has this big source of media (publication) at their fingertips.
Yes, it could very well be that "valid" email may soon drop to near nothing in the future.
Hope this helps to shed some light on the topic.
I keep hoping that the IPv6 IP system will be put into effect that would give all IP numbers sort of a "country code prefix" that could be used to stop a lot of foreign spam.