Before sharing my email address with some person or some org, I do an MX DNS lookup on the domain portion of their email address. It’s usually correct. That is, if the result is not of the form *.mail.protection.outlook.com, then that recipient is not using Microsoft’s mail server.
But sometimes I get stung by an exception. The MX lookup for one recipient yielded barracudanetworks.com, so I trusted them with email. But then they sent me an email and I saw a header like this:
Received: from *.outbound.protection.outlook.com (*.outbound.protection.outlook.com…
Is there any practical way to more thoroughly check whether an email address leads to traffic routing through Microsoft (or Google)?
You’re seeing that behavior because some companies may have mailboxes in M365 but use a different provider for message hygiene, such as Barracuda, Proofpoint, MX Logic, etc. The MX points to them, they forward to an M365 inbound connector (virtual MTA) after inspecting the email.