When a subscriber is using Hotmail, Yahoo!Mail or TDC Webmail, and marks your newsletter as "Junk", Ubivox receives a notification stating that a subscriber has "complained" about receiving the newsletter. The subscriber is then, automatically, suspended from the list. At the same time, the complaint is visible in the statistics overview of the newsletter.
A complaint is considered more serious than an unsubscription. Often a complaint level above 0.3%, is high, and you should consider whether your newsletter content or the information about why the subscriber is receiving your newsletter should be better.