Taughannock Networks Flexible Sender Validation (FSV) is a lightweight validation technique to detect and deter some kinds of e-mail address forgery. It publishes information in the DNS about IP addresses authorized to send mail for a domain, one in a family of IP based mail validation proposals dating back to Paul Vixie's original in 2002[5]. FSV uses redundant copies of IP data to permit both efficient use by very high-volume mail servers, and simple implementation on low to moderate volume mail servers.