Spam filters generally identify offenders by the e-mail source or content, and block about 20% of e-mail marketing messages, notes Bilbrey. Customers have a big say on what is spam: If the ratio of complaints to messages exceeds a certain threshold, often 1% to 2%, most ISPs will tag the sender as a spammer.

E-mail can also be tagged as spam when it is sent to spam traps (unused addresses that ISPs maintain just for this purpose). Even legitimate senders can unwittingly send to a spam trap, for example, if customers opting in for an offer have mistyped their e-mail address.