The "hop count" in networking refers to the number of devices a particular network packet needs to move through to reach a given destination. In e-mail, the hop count refers to the number of mail servers a message passes through on its journey from the sender to the recipient. The "Received:" header of an e-mail message contains a breakdown of the total route traversed by the message, so a hop count can be derived by counting the total number of hosts listed there.
Hop counts are used in e-mail to stop problems with mail loops. For instance, if email@example.com is set to forward to firstname.lastname@example.org, and email@example.com is forwarding back to firstname.lastname@example.org, the hop count will enumerate how many times a message is passed back and forth, and will eventually stop the message from "looping" endlessly between the two addresses. A loop problem will usually be evident in a message header. Any message that runs out of hops will be returned with the error 554: hop count exceeded (or a similarly-worded message).
Each version of Exchange has a different hop count. The default hop counts for each are usually set high enough that messages can traverse even fairly complicated e-mail systems without problems, but still be able to stop mail loops. That said, there are some mail systems that add large numbers of hops to a message due to internal relaying, so it may at some point become necessary to change the hop count to insure that messages arrive safely.
Exchange 5.5 has a default hop count of 18, while Exchange 2000's hop count is 15. By contrast, Exchange 2003 has a default hop count of 30 (which is the highest most administrators would ever set it).
If you want to raise the hop count in 5.5, edit the Registry key HKEY_LOCAL_MACHINE SYSTEM CurrentControlSet Services MSExchangeIMC and set its value to the number of hops to use, in hex. In Exchange 2000 and higher, the hop count can be set by going to the SMTP Virtual Server's Properties panel. Under the Delivery tab, click the Advanced button and edit the Maximum Hop Count value.
Serdar Yegulalp is the editor of the Windows 2000 Power Users Newsletter.
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