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?
I see. When you wrote :
I assumed that the customers were sending it from GSuite based custom domains, but this is plain gmail.com and that makes the DMARC part different. Did your company make any DNS changes just before this happened ? SPF records have a maximum length they can have. If it exceeds the maximum length, it can cause failure and email rejections.
A G Suite account was the first thing that I thought of as well, but no, this is straight gmail.com. I actually tested with my personal G Suite account, and I got similar results. Because I’m in the support team for the manufacturer, we’ve been hearing reports from a large number of customers who have been complaining of this over the past 48 hours.