Gmail now sets hard rules for every sender
Since February 1, 2024, anyone sending to Gmail accounts must set up SPF or DKIM and keep the spam rate reported in Google Postmaster Tools below 0.3%. Google recommends staying below 0.1%. Senders of more than 5,000 messages a day to Gmail must also set up DMARC and support one-click unsubscribe for marketing messages. A handful of complaints per thousand emails is enough to start landing in spam.