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.