The F=R delivery agent
flag causes sendmail to connect on a reserved
(privileged) TCP port for additional security. Privileged Internet
ports are those in the range of 0 to 1023. Only
root is allowed to bind to such ports. The
sendmail program calls
rresvport(3) to obtain a socket on its selected
port. Note that this is done only when instantiating an outgoing
connection.
This delivery agent flag is suitable only for use with the
A=TCP delivery agents. Note that
sendmail is usually started by
root when run as a daemon.