we have 97% reputation on sendgrid, and the only mail we send are a) customer requested password reset and b) customer requested notification for completed workflow jobs.
I don't know who that 3% is, but it is enough to get sometime filtered and I don't think we can ever win this
The 3% may very well be ignorant users who are using the Junk button as a way to remove the mails from the inbox (yeah, people actually do that), but for good measure, things like signup forms and password reset forms really should be protected with a captcha, as any form able to send any form of email whatsoever will be abused by bots trying to send spam.
so is gmail by the way.
I'm in favour of anti-trust action against Google and Microsoft for their outright anti-competitive anti-spam practices, but that's going to be a long and hard up-hill battle for anyone attempting it.
Meanwhile, the rest of us will have to accept things for what they are and deal with them as well as we can.
well we do have a user existence checks
As far as I can tell, it simply is not possible to buy a new domain, pay for a email service and have mails delivered straight into people's inboxes. After all, if you would be able to do that then so could the spammers.
Ramp up volumes slowly. Make sure the recipients actually want the mails you send, and for all that is holy, educate your users that the "Junk" and "Trashcan" buttons are two very different things. People who repeatedly mark your mails as spam need to be blacklisted, no matter if they're paying customers. Period.
Oh, and handle abuse complaints right away. Don't be afraid to tell people they're not supposed to mark your mails as spam and to explain the consequences.