What's your setup? If you're forwarding, an SPF-only aligned email will fail DMARC, and it's the sender's fault: https://news.ycombinator.com/item?id=45441243

Next time a legit email ends up in your spam folder, use this tool to figure out why: https://mxtoolbox.com/EmailHeaders.aspx

I've had a few cases myself, and it's always been the sender's fault.