If the hop count for a message (i.e. the number of hosts
                already traversed) equals or exceeds this number, attempts  at
                remote delivery will result in an error message being returned
                to the sender. This is used to prevent messages  from  looping
                forever.  The  hop count is generally computed from the number
                of Received: fields in the mail header, but may  also  be  set
                manually using the -h option on the command line.
                     This variable defaults to 20.