I work for the support department of a large multinational imaging company. Starting yesterday, we started getting tons of calls from customers who have been sending email from their devices from Gmail domains who are not able to send emails to M365 users. A bit of snooping in our test M365 domain shows that they are being dropped as spam from M365. What’s odd is that I cannot find any mention of this behavior anywhere on the Internet. Has anyone else seen this yet?
That’s a great suggestion, but the message gets tossed by the M365 spam filters before it reaches the inbox and I never have a chance to see the headers. All I’m able to see is the snippet of information that doesn’t show anything actionable.
Are you using defender for o365? If so, usually uou can get a copy of Any rejected messages at security.microsoft.com and get more info from the message explorer