We have successfully setup feedback loop with mailtrust.com.  Mailtrust is a business email hosting provider and are currently hosting over 900,000 mailboxes.  With this setup, any complain received from mailtrust users will be forward to us automatically allowing us to take appropriate actions.

What is a feedback loop?

A feedback loop (FBL), sometimes called a complaint feedback loop, is an inter-organizational form of feedback by which an internet service provider (ISP) forwards the complaints originating from their users to the sender’s organizations. – Wikipedia