Complaints can occur because users, in many e-mail programs, can report an e-mail as spam by clicking a special button.

These so-called feedback loop complaints are anonymously transmitted to promio.connect by providers, such as Hotmail, Yahoo, AOL, and 1&1, every time a receiver marks a mailing as spam. Depending on the unsubscribe method selected, the user is added to the email blacklist or, if they are in use, added to the corresponding block list or unsubscribed from the newsletter.

Complaints can lead to a negative reputation for your sender domain and thus to worse deliverability.