It's common knowledge that Microsoft considers everyone to be spammers until proven innocent. Getting to a point where your mails end up in the recipients' inboxes is a long path -- Microsoft doesn't care if you set everything up perfectly, as spammers can do the same -- and one has to provide them with enough data to learn to trust one's IP's/ranges/domains.

Contacting Microsoft means you have to go through a difficult dance through multiple canned replies, multiple escalation requests, and being lucky enough to get someone to actually escalate.

Silently dropping emails isn't Microsoft-specific, though. It's quite common to have a Postfix/Amavis setup set to discard spam over a certain spam score threshold.