In the MediaPost article "Why 2010 Could Be A Bad Year For 'Worst-Practice' Marketers," ReturnPath's George Bilbrey talks about what's driving ISPs nuts lately: legitimate companies with bad email practices. He writes: "The good news is that the ISPs (and their technology providers) are doing a much better job at preventing much of the truly criminal spam. With the worst mail out of the way, what are they finding? Of the mail that is not criminal spam, the mail streams that are causing the most noise from ISP subscribers (high "this is spam" rates, high spam ratings from spam rating panels, low "this is not spam" rates) is mail coming from legitimate companies with very poor practices. These mailers are now front and center on the ISPs' radar screens, which will result in widespread, critical delivery problems for this class of mailer in 2010."
This is something I've witnessed first hand, and I've also seen what happens next when ISPs denote that these mail streams aren't all that kosher: They block. Hard block, permanent block, spam block. The anvil drops right on the sender's head, and it takes months to clean things up and get unblocked. I've seen at least one really big ISP say to heck with this, don't ask us to unblock it until six months have passed AND you've cleaned up your act.
This is all yet another data point on why an ounce of prevention is much better than a pound of cure. Clean your act up now, so you keep getting to the inbox, instead of waiting until you get blocked and suddenly have to scramble to try to rescue your email program.