Spam is back with a vengeance and the effect on ISPs and other email access providers is substantial, requiring ever-growing resources just to avoid drowning in the flood.
In response, ISPs are doing what they've always done: tightening delivery requirements. The good news for email marketers is the use of image spam and extreme variations in content mean that content filtering is less of a focus. The bad news is list hygiene and other chores related to best practices are becoming much more onerous. In particular, ISPs are increasingly aggressive about bounce rates. There's a clear correlation between high bounce rates and spam, so it's understandable ISPs utilize this as part of their defense.
This means marketers must now remain within the bounce rate requirements at all times, making older list cleanup much more difficult. In recent months, some ISPs have become so aggressive about bounce rates that even legitimate communications preferences are no longer tenable.
Unless and until ISPs realize they've tightened requirements so far that some classes of legitimate email simply can't be delivered within the parameters they've set, it will be very difficult to resolve this issue.