Guilt by tenuous unwarranted association
Practically all the traffic we received from Digital Ocean and Linode was garbage, so we blocked all their networks.
It is likely because some moron at M$ thinks like this that my email to places that have been overrun by M$ (significantly including many universities) doesn't get through.
I regard it as my Civil Right in as a citizen of Cyberspace to have my emails handled independently of the tech giants.
It is sufficient for my level of usage to use shared hosting for my email and website. That is done by Primexeon in Cambridge, whose delighted customer I have been for 18 years.
They in turn use the cloud services of Linode.
However, M$ scrambles the standard email headers and replaces them with hundreds of lines of their own garbage. In some inscrutable way they they judge a large fraction of the legitimate email that passes through them as spam.
I had the same problem with Google/gmail, which I apparently solved by routing my outgoing email though my phone/broadband provider, ICUK/CIX, of whom I am also a longstanding happy customer.
Neither Primexeon nor ICUK/CIX have any idea how to fix this. If I say "debugging" and "M$" in the same breath, I can't keep a straight face.
On what basis M$ judges my email to be spam I have no idea. If it's not the IP address then it's presumably the domain name of the From: address.
But the most likely explanation is that some j*rk at M$ thinks it's a good idea to block everything that has a Linode MX record, even though it hasn't come from there and they're not sending anything there.