@jab1
The root of the issue is indeed the alignment of what is deemed to be a bulk sender. In the various reference documentation on mitigating the propagation of spam, there are very firm recommendations to segment email into sub domains, so that email reputation of one sub domain does not damage the reputation of either the root domain or other sub domains.
Then there is orthogonal documentation from Microsoft which states that all sub domains of a root domain will for bulk sender aggregation be treated as a single bulk sender. Thereby there is an incomprehensible contradiction.
Similarly, if I understand the details of the empirical experiences reported around here, emails sent to individual Microsoft addressees arrive as expected - no verification issues. However if the same email is sent to the same addressees as a group, some Microsoft environments start raising DKIM failures, reported as lookup failures, not DKIM mismatches.
That is consistent with the widely reported Microsoft DNS performance issues and differing SCL metrics for the same email in different target environments. SCL is the Spam Confidence Level.
Why differing Microsoft regimes should treat the same email / addresses differently in the various circumstances is something which only Microsoft can explain. The in-flight dialogues should result in Microsoft explaining their presently incomprehensible variable behaviour.
...if only Microsoft were as proscriptive in stopping their services being the source of spam!
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