A few years ago, I was dealing with a few small business sites which were self-hosted and always had deliverability problems with the large mail services (mostl

A few years ago, I was dealing with a few small business sites which were self-hosted and always had deliverability problems with the large mail services (mostly, back in the day, AOL and Yahoo). Large providers tended to have a world view dividing all senders into two categories: 1) Bulk senders who are clearly mailing the same spam to a list of a billion addresses 2) Non-commercial individual hosts which should be sending five messages a day or less in total. It felt like there was a huge missing third category for transactional emailers nobody wanted to acknowledge. They are probably difficult to score fairly. A hundred "Order details" emails are going to have the same level of randomness/templatedness as the old Viagra spam which had a random block of Project Gutenberg text pasted at the end to trip up filter math. You're not going to have a clear history of "this address bounced twice, let's stop sending newsletters" when most of your messages are to first time customers or once-every-few-years return ones. A lot of the messages will look generic because they use default shopping cart templates. To the extent they provided sender guidance, it was focused around use case 1) -- sign up for feedback loops and deal with greylisting (because people really love waiting 18 hours for an acknowledgement)