[nuccy]

Moreover they don't even whitelist their own IPs for some basic checks like SPF, which can be skipped. I have a work email (using Microsoft services) and another company mailing list, which my email is member of (also on Microsoft). At some point sending email to that mailing list triggered bouncing between Microsoft own servers eventually resulting in my email being automatically removed from mailing list. Basic investigation showed that one Microsoft server rejected emails from another one because of bad SPF record. Either company spent months solving the issue with Microsoft. The issue disappeared eventually magically the same way it appeared.