Would you mind sharing where you host your email server?
I have a pet theory that big email platforms care deeply about the IP "neighborhood" the sending email server lives in. So a small email server set up perfectly in Digital Ocean IP space would struggle, while an email server set up OK in (for example) CenturyLink Enterprise IP space would get through.
Hmm, I just tried what you suggested, with 2 different accounts never sent to, and the emails got through fine.
We use SPF and DKIM, but otherwise it's a fairly standard Postfix/Dovecot install (albeit a very old one).