A Feed Back Loop (FBL) is a direct data feed that is sent to an email sender when the recipient presses the "Junk" or "Spam" button. The benefit is that the email sender can clean their subscriber data, allowing them to maintain their reputation with Internet Service Providers.
What triggers (flags) cause ISPs to block your email? Sometimes it's a dirty list containing non-deliverable addresses. Other times it's the ratio of spam complaints from that domain, which can include false positives.
Here are the grades for ISP's where legitimate messages were flagged and determined to be false positives: Yahoo! gets an A, Hotmail B, AOL C, and Gmail scores the lowest with a D.
Some techniques to avoid false positives are:
As always--be aware of your list.