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)?