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Delivery problems are not all spam related

Not every delivery failure is due to poor reputation or spam. Sometimes ISPs just have problems on their mailservers and so mail doesn’t get through. It’s often hard for delivery experts (and their bosses and their customers and their clients) to watch email delays or rejections without being able to do anything about it.
Sometimes, though, there is nothing to do. The rejections are because something broke at the ISP and they have to sort through it. Just this week there’s been a lot of twitter traffic about problems at a major cable company. They are rate limiting senders with very good reputations. They have admitted there is a problem, but they don’t have a fix or an ETA. From what I’ve heard it they’re working with their hardware vendor to fix the problem.
Hardware breaks and backhoes eat fiber. Yes, ISPs should (and all of the large ones do) have backups and redundancies. But those backups and redundancies can’t always handle the firehose worth of mail coming to the ISPs. As a result, the ISPs start rejecting some percentage of mail from everyone. Yahoo even has a specific error message to distinguish between “we’re blocking just you” from “we’re shedding load and temp failing everyone.”

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Freemail opens

Justin Coffey commented on my check your assumptions post pointing out his data on opens related to ISPs. He says:

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Why do ISPs do that?

One of the most common things I hear is “but why does the ISP do it that way?” The generic answer for that question is: because it works for them and meets their needs. Anyone designing a mail system has to implement some sort of spam filtering and will have to accept the potential for lost mail. Even the those recipients who runs no software filtering may lose mail. Their spamfilter is the delete key and sometimes they’ll delete a real mail.
Every mailserver admin, whether managing a MTA for a corporation, an ISP or themselves inevitably looks at the question of false positives and false negatives. Some are more sensitive to false negatives and would rather block real mail than have to wade through a mailbox full of spam. Others are more sensitive to false positives and would rather deal with unfiltered spam than risk losing mail.
At the ISPs, many of these decisions aren’t made by one person, but the decisions are driven by the business philosophy, requirements and technology. The different consumer ISPs have different philosophies and these show in their spamfiltering.
Gmail, for instance, has a lot of faith in their ability to sort, classify and rank text. This is, after all, what Google does. Therefore, they accept most of the email delivered to Gmail users and then sort after the fact. This fits their technology, their available resources and their business philosophy. They leave as much filtering at the enduser level as they can.
Yahoo, on the other hand, chooses to filter mail at the MTA. While their spamfoldering algorithms are good, they don’t want to waste CPU and filtering effort on mail that they think may be spam. So, they choose to block heavily at the edge, going so far as to rate limit senders that they don’t know about the mail. Endusers are protected from malicious mail and senders have the ability to retry mail until it is accepted.
The same types of entries could be written about Hotmail or AOL. They could even be written about the various spam filter vendors and blocklists. Every company has their own way of doing things and their way reflects their underlying business philosophy.

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