I am sending single letters of notifications, yet they are being marked as spam by almost every server.
I've got detailed report from "is not spam" service, but can't really understand what lines of it are important. From what I have gathered by googling, everything looks moderately OK, yet emails end up in spam anyway.
Please tell me, what am I missing.
To send email and have it not marked as spam you need to ensure a few different things.
With all of this your messages can still be marked as spam if the content of them looks suspicous.
Amazon has a service SES that I have been using for sending emails, they make it really easy to setup most of this stuff and so far I haven't had any emails sent via that marked as spam.