"ISPs are now moving toward customer-level adaptive learning filters and placing explicit and learned customer-level behavior over all other levels of filtering (with the exception of outright Port25 blocks).
This is an incredibly important shift with huge implications for mailers. It means, for example, that it does not matter if you have 0 complaints and 0 unknown user rates – if customer X has negatively engaged with your mail (e.g. historically ignored your mail, moved your mail to the spam folder, etc), it will be placed in the spam folder for that customer.
Conversely, if your spam complaint rates and unknown user rates when computed at the IP or Domain level are such that you’re designated as spam, your mail will still be placed in the inbox for those customers who have positively engaged with your email (e.g., opened, clicked, replied, moved you to inbox, personal folders, reported you as not spam, added you their address book, etc)."
A number of top ISPs are putting in place, or have already established, domain-based reputation systems. It is a major shift in how email messages are delivered - one for which email marketers clearly need to prepare.
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