When a message is delivered, it may be routed to a number of servers before reaching its final destination. You can designate how many servers the message is allowed to pass through. This is referred to as the hop count.
After the hop count is set, the SMTP server counts the hops listed in the Received lines of the message header. When the number of Received fields exceeds the maximum hop count setting, the message is returned to the sender with an NDR.