Spam folder

What it means

Accepted mail that a provider's filter distrusts goes to the spam folder instead of the inbox. Gmail calls it Spam; Microsoft's documentation uses Junk Email folder, and its anti-spam policies deliver bulk mail at or above the configured bulk complaint level there by default. Recipients move mail in and out themselves: reporting a message in Gmail adds it to Spam, and Google says that once a recipient marks a valid message as not spam, future messages from that sender stay out of Spam.

For senders the spam folder is a silent failure. The server accepted the message, so there is no bounce, and Gmail's Postmaster Tools notes that mail Gmail files in spam automatically is not counted in the user-reported spam rate.

Providers do not report folder placement per message, which is why seed tests and engagement trends are used to detect it.