Did you have spf and dkim configured for your server ?
Tobias Oetiker

Yes. DMARC as well. I guess you could argue that IP-based reputation is going to become less and less important, considering that domain-based reputation provides a better basis for reputation assessment. Indeed, that is already the case for IPv6, for example: most mail receivers have defined that you need some sort of authentication when sending via IPv6, otherwise they will reject your mails.

The current reality is, however, that IP-based reputation is still very much in use, and reputation-based filtering is becoming more and more the principal measure against spam.

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