On 13 January 2024 14:07:46 "L. Mark Stone via mailop" <mailop@mailop.org>
wrote:
Is there a list of "legitimate" subdomains of onmicrosoft.com somewhere
that we can leverage?
Wearing my "I have to administer a Microsoft 365 tenancy" hat - no.
However, your mention of best practice is bang on. The subdomains of
onmicrosoft.com are tenant boundaries and not intended to be used for
email. Domains should be added, verified and configured properly for
outbound mail.
I would personally say that you will lose practically no real email by
rejecting those subdomains completely - and if you get complaints from
actual M365 tenant customers, point them at the docs.
Graeme
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