When someone hits "mark as spam," it's the loudest negative signal a subscriber can send — and mailbox providers listen. A rising complaint rate can quickly push your mail to the spam folder for everyone.

How is spam complaint rate calculated?

Spam complaint rate = (spam complaints ÷ emails delivered) × 100. Google's sender guidelines require keeping it below 0.3%; above that, more of your mail lands in spam by default.

What causes high complaint rates?

Emailing people who didn't opt in, sending too often, unclear sender identity, misleading subject lines, or a hard-to-find unsubscribe link.

How do you keep complaints low?

Use permission-based opt-in, make unsubscribing easy, set expectations at signup, segment for relevance, and honor a sensible sending frequency.