On the other hand, most human-generated mail is HTML-based, as it's written in a web-client or desktop/mobile "app client", that is using HTML to let people do bold and put pictures into their email.
I was as surprised as you when I figured this out!
I'll have to give this a try. Thanks for the tip!
It was the same issue - even replies to existing threads had the issue. What got me looking into it more closely was that I had no issues delivering to G Suite tenants; it was only issues with 365 tenants.
I wonder if there's a non-trivial correlation between people choosing to genuinely self-host their own mail, and those who use plaintext outbound mail by default (and know what that means).
Are you sure there is not some sort of parasitic Bayes filtering happening here?
I never found much by way of technical detail on 365 deliverability, beyond the usual DKIM/SPF/DMARC/PTR record guidance, and forms for getting yourself removed from the whitelist.
What's especially strange is that this issue applied even when sending mail as a 365 subscriber. Once there was HTML content present in the email, deliverability (to other separate 365 tenants) improved!
Very odd...
For anyone with deliverability issues and mail going to spam, it might be worth trying this out (after you have sorted all the usual points like DKIM, SPF, PTR record etc.) - this obviously only helps if your IP is clean. I was surprised though, and only discovered it by accident.
I had good deliverability to Google hosted mail, but not to Microsoft 365. I even had issues with deliverability doing 365 to 365, until I sent HTML mail.