How does Gmail decide an email is spam?

Last reviewed: October 21, 2026 · Markdown version

Google does not publish how its spam filter weighs a message, but it tells you three things: why it moved each message, how your own clicks train it, and what senders have to do to stay out of Spam. The first is on the message itself. Google's spam page says that in Spam you will "Find a warning at the top of each email that explains why Gmail sent it to Spam" (Report spam in Gmail, read 21 October 2026). Open any message in your Spam folder and that banner is Gmail's reason, in its own words.

The reasons Gmail gives

The same page lists the warnings you can see, with what each means:

Spoofed email addresses. "The email address looks very similar to the email address of a known sender", for example a zero in place of the letter O.
Phishing scams. The message "may be a trick to get you to share personal information, like passwords or credit card numbers."
An unconfirmed sender. "Gmail can't confirm who sent you the email." This is the authentication check described further down.
Administrator-set policies, on work or school accounts, where "your admin might set controls to mark certain emails as spam."
You tried to unsubscribe. "If someone sends an email after you unsubscribe, their emails go directly to Spam."
Empty content. Spammers "often send emails with no content in the body or subject" to check which addresses are live.
You sent it there. After you report a message, "Emails from the same sender might be sent to the Spam folder in the future."

What the page does not say is how the filter scores mail that carries none of these marks, or how much each signal counts. The list is what Google chooses to show you, not a description of the machinery.

Your clicks teach it

Google's page opens with the part you control: "As you report more spam, Gmail identifies similar emails as spam more efficiently." It works the other way too. Clicking Not spam on a message means "future emails from that sender won’t go to Spam", and adding a sender to Google Contacts makes Gmail stop sending their messages there. A block overrides both: "When you block a sender, even when you remove their emails from Spam, Gmail still automatically identifies their emails as spam." So a newsletter you want that keeps landing in Spam usually needs a Not spam click and a contact entry, not a new setting. More on what Spam keeps, and for how long.

What Google asks of senders

The other half of the decision is about the sender, not the message. Google's guidelines for anyone mailing personal Gmail accounts set requirements meant "to help prevent Gmail from limiting sending rates, blocking messages, or marking messages as spam" (Email sender guidelines, read 21 October 2026). Among them, for all senders: set up SPF or DKIM authentication, and keep "spam rates reported in Postmaster Tools below 0.3%". Senders of more than 5,000 messages a day must also set up DMARC, and their marketing mail "must support one-click unsubscribe". The same page explains why your reports matter to the sender: "If messages from your domain are frequently reported as spam, future messages from you are more likely to be marked as spam."

In plain terms, a message that cannot prove where it came from, from a sender whose mail many people report, is the kind Gmail's own documents point towards Spam. The guidelines also rule out a shortcut for email services: "Google and Gmail don’t accept allowlist requests from email providers."

Why spam can suddenly surge

One warning on Google's list explains a sudden flood rather than a single message. Under "Spam attack on your Gmail account", Google says that when you get many unwanted emails, "like subscriptions or promotional offers, a hacker tries to fill up your Inbox" so that you miss real security alerts from services you use. Its advice is to search your inbox and Spam for those alerts, run a Google Security Checkup, and follow its security tips (the Report spam in Gmail page above). If what you are seeing is a rise in fake login or payment emails instead, that is phishing, which has its own page.