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?
M365 is notorious for randomly deciding that valid emails are spam and there’s no way to redress it. Welcome to hell.
One thing i can suggest if smtp is tripping and web aren’t - check your sig. we got marked as spam for having http://ourdomain instead of https.