I've run into similar recently with delivering to our own company's email boxes from our own server - company uses office365 / msoft business(?)

after jumping into dkim spf and all that, I can get delivered from our server to gmail.. but not to the company outlook boxes.. tried to get [third party corporate IT] to whitelist the ip or domain - they cant find receive attempts in logs..

back and forth, showing screenshots with timestamps, paying stupid money per hour to [NotNaming].. get told that well their msoft thing depends on AWS as a middle man and it is hardcore about spam stuff and if that's the problem it will take a ton of 3 team digging?

Giving up - now trying to find a way to have the server send an email to gmail and then forwarding to company's office365 - maybe, I dunno yet.