A hop is the transmittal of a mail message from one machine to another.^[37] Many hops might be required to deliver a message. The number of hops is determined by counting the Received:, Via:, X400-Received, and Mail-From: lines in the header of an email message.^[38]

^[37] The IP transport protocol also has the concept of hops. A message going from one machine to another has only one mail hop but can have many IP hops.

^[38] Actually, any header that is marked with an H_TRACE flag (Section 25.6.17) is counted.

The MaxHopCount option tells sendmail the maximum number of times a message can be forwarded. When sendmail receives a message via email, it calculates the hop count. If that count is above the maximum allowed, it bounces the message back to the sender with the error:

sendmail: too many hops (17 max)

In this case, 17 is the maximum. Detecting too many hops is useful in stopping mail loops—messages being forwarded back and forth between two machines.

The forms of the MaxHopCount option are as follows:

O MaxHopCount=hops        configuration file (V8.7 and later) 
-OMaxHopCount=hops        command line (V8.7 and later) 
define(`confMAX_HOP',hops)    mc configuration (V8.7 and later) 
Ohhops                    configuration file (deprecated) 
-ohhops                   command line (deprecated) 

The hops argument is of type numeric. If hops is missing, the value becomes zero and causes all mail to fail with the error:

sendmail: too many hops (0 max)

If the entire MaxHopCount option is missing, hops defaults to 25. A good value is 50 or more (RFC2821, Section 6.2, suggests 100). This allows mail to follow a fairly long route through many machines (as it could with UUCP) but still catches and bounces mail caught in a loop between two machines.

The MaxHopCount option should not be confused with the -h command-line switch (-h). The MaxHopCount option specifies the maximum number of hops allowed, whereas the -h command-line switch presets the (beginning) hop count for a given email message.

The MaxHopCount option is not safe. If specified from the command line, it can cause sendmail to relinquish its special privileges.