Complaint

A complaint is what a recipient generates by pressing "report spam" or "junk". The mailbox provider reports it back to the sender through a feedback loop, and it is the single most damaging signal in email.

The damage is not to that one message. It is to your sender reputation, and therefore to every message you send afterwards, to everyone.

The number to know

A complaint rate above roughly 0.1% — one in a thousand delivered messages — is treated as a problem by most major providers. That is a much smaller number than it sounds: on a 50,000-message campaign it is fifty people.

Filtering usually begins well before anyone tells you it has. By the time deliverability is visibly bad, the reputation damage has already been done and takes weeks of good behaviour to undo.

What causes them

Almost always one of three things, and almost never the content itself:

• The recipient does not remember subscribing. See double opt-in.
• The unsubscribe link is hard to find, so the spam button is easier. See List-Unsubscribe.
• The mail is not what they expected — a different frequency, or a different subject, from what they signed up for.

In Sendly

Complaint notifications are processed automatically and the complaining address goes onto your suppression list immediately. Complaint events also reach your webhooks.

Sending mail to an address that has complained is not something you can opt out of being stopped from doing, and that is deliberate.