- a clean IP, not on RBL, not blacklisted elsewhere to the best of my knowledge (outlook.com tells you when you IP is bad)
- also accessible on IPv6
- both IPs having a proper rDNS on my domain
- supporting SSL on port 487 and 565, with a certificate from a known authority
- with DKIM and SPF both passing according to gmail
Yet it ends up in gmail spam folder. And I'm only sending email to myself and 2 other persons, so it's not even mass mailing.
I think there are other factors at play.