'Retail' versus 'wholesale' spam
A while back I mentioned that the spam received by my spamtrap SMTP server is boring; it's mostly advanced fee frauds, phishes, and the like. In light of that and that GMail based people keep trying to send me spam, I've been thinking about how one way to split up spam is between what I'll call retail spam and wholesale spam.
Wholesale spam is the high volume emitters, the people who are doing it in enough volume that they have real infrastructure and automation of some sort. These are the 'email marketing' people and the people who wind up on the SBL and so on and so forth. The modern problem for them is that their very volume makes them recognizable and thus blockable. We have DNS blocklists, we have spam feature recognition in filtering systems, and so on and so forth. As a result of this, I think that wholesale spam is a mostly solved problem for most systems.
Retail spam is the small volume and often hand entered stuff. It is people sitting in Internet cafes using stolen webmail credentials to send out more or less hand-written messages. This is the domain of a great deal of advance fee fraud and phish spam, and as a result of its comparatively small volume and hand done nature it's hard to do a really good job of blocking it today. It's probably always going to be hard to fully block this, and as a result I can unhappily look forward to GMail emitting this stuff in my direction for years to come.
(GMail is far from alone here, of course; any freemail service is a sending source for this stuff. I just notice GMail more than the others for various reasons.)
Maybe someday we'll figure out really effective tools against retail spam, but I doubt it. Stopping retail spam runs up against the fundamental problem of spam.