How do I know who complained?

Mailbox providers use Feedback Loops (FBLs) to send a report back to your Email Service Provider (ESP) when a user complains. Your ESP will then suppress the address.

Are spam complaints worse than hard bounces?

Yes. Spam complaints are a direct signal to ISPs that users actively dislike your mail. High complaint rates will quickly route your future emails to the spam folder.

Spam Complaint — what it means and how to handle it

Meaning

A spam complaint occurs when a recipient actively clicks the 'Mark as Spam' or 'Junk' button in their email client.

Common causes

The recipient did not opt-in to receive your emails.
The recipient found your content irrelevant, too frequent, or annoying.
The unsubscribe process was too difficult, so the user marked it as spam instead.

How to fix

Immediately remove the complaining user from your mailing list. Your ESP usually handles this automatically via a Feedback Loop (FBL).
Ensure your emails have a clear, easy-to-find unsubscribe link.
Send relevant, expected content to engaged subscribers only.

Provider notes

Amazon SES. SES strictly monitors complaint rates. A complaint rate above 0.1% can trigger a warning, and exceeding 0.5% risks account suspension.

Example bounce

N/A (Complaints are reported via Feedback Loops, not SMTP bounces)

FAQ

How do I know who complained?: Mailbox providers use Feedback Loops (FBLs) to send a report back to your Email Service Provider (ESP) when a user complains. Your ESP will then suppress the address.
Are spam complaints worse than hard bounces?: Yes. Spam complaints are a direct signal to ISPs that users actively dislike your mail. High complaint rates will quickly route your future emails to the spam folder.

Spam Complaint User marked as spam