News: Gmail doesn't like that you didn't comply with our policy

All mail providers are seeing an increase in rejected emails from Gmail with the following message:

"Our system has detected that this message is likely unsolicited mail. To reduce the amount of spam sent to Gmail, this message has been blocked. Please visit https://support.google.com/mail/?p=UnsolicitedMessageError for more information."

Every customer who has noticed this and opened a ticket with us has one thing in common: They did not comply with our policy and set a proper SPF record in their DNS. Easy fix, right? Read up: https://mxroutedocs.com/dns/dnsrecords/

Speaking with other mail providers, others are seeing the same change. Pulling numbers for emails rejected with this error by day, it appears that Google made a significant change to their spam filters late into the day on June 6th:

June 1:
50
June 2:
70
June 3:
56
June 4:
30
June 5:
50
June 6:
368
June 7:
2710
June 8:
2198

This doesn't correlate with any change in activity for our customers, the increase isn't simply because some of our customers started large mailing campaigns on that day. This is very organic from our side.