http://boxofmeat.net/post/34454023

For additional perspective, though, I should add that it’s overly simplistic to claim that ’services such as Hotmail and Yahoo “trust” Gmail.’ Anyone doing reputation calculations will quickly see that you can’t hold ISPs & freemail providers to the exact same thresholds as email marketers: marketers control their entire mailstream, while ISPs have end users. ISPs will kick off end users who do bad things, while most marketers will keep sending until something external forces them to stop.

The thresholds for entirely unknown IPs tend to be even lower, because most of those are botnets sending the worst kinds of spam.

For years, spammers have been creating freemail accounts en masse and then sending through them to other ISPs & freemail providers. It can be difficult to for the ISPs to control, because they don’t want to harm their real users.

I would strongly urge anyone in the email marketing industry to pay attention to spammers’ tactics, both to avoid acting like spammers and to understand why ISPs react the ways they do.