sasl_checkapop() will check an APOP challenge/response. APOP is an optional POP3 (RFC 1939)
authentication command which uses a shared secret (password). The password is
stored in the SASL secrets database. For information on the SASL shared
secrets database see the
System Administrators Guide in the doc/ directory of the SASL distribution.
If called with a
sasl_checkapop() will check to see if the APOP mechanism is enabled.