Complaint rate is the percentage of recipients who flag your emails as spam.

When you divide the number of spam complaints by the number of emails delivered, you have the complaint rate. This is the rate depicting how often your subscribers register their displeasure with the mailbox providers for getting your emails. Mailbox providers estimate the complaint rates by dividing the number of complaints by the number of messages sent to the subscribers’ inboxes.

Unwanted mail can now generate complaints when messages hit the junk folder at different mailbox providers. Every time your contacts mark your messages as “Junk”, it’s factored into your overall complaint rate, which considers the number of messages marked as junk, over the number of emails sent in a specific email program. Best practices tell us that this complaint rate should be less than 0.1% (one-tenth of one percent) with each ISP, so if your complaint rate is much higher than this, consider going back to your list and check for contacts with low engagement for future email sends.