The mail gopher (mopher) is an extension to all mail transfer agents that implement the Sendmail Milter API. It reads a list of rules and acts on each incoming mail delivery attempt based on various criteria and existing states. Mopher supports indiscriminate and selective grey- and whitelisting of either individual addresses or whole domains (based on reverse lookups and rules provided by Mozilla's Public Suffix List). Access to tables and state-related data passes through a generic interface and is then translated by one of the available backend drivers.