After a slight lull, spam is back with a vengeance. Botnets (define) are growing in size and sophistication, enabling spammers to send more e-mail than could have been conceived of only a few years ago. This virtually unlimited capacity to send spam allows image spam and pump-and-dump stock spam to grow at an enormous rate. The ability to send tens, even hundreds, of message variants to the same recipient is very successful at getting through filters. The effect on ISPs and other e-mail 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 e-mail 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.

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