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A common and deeply annoying side effect of spam is bogus non-delivery reports (NDRs).
If Exchange Server receives an e-mail sent to a nonexistent address (possibly as part of a dictionary attack strategy), it will attempt to fire off a non-delivery report to the target server.
However, if the target server doesn't even exist -- i.e., if it's a synthetic or randomly generated domain name like xqmngbtrd.com -- the NDR will never reach its intended target.
There's an even further complication: if you receive, say, a thousand spam e-mails from a bogus domain, the consequent NDRs will sit in Exchange Server's outgoing message queue and create DNS lookups for every message.
For bogus domains, DNS lookups will time out. Those excessive DNS lookups will also likely create too much UDP traffic to your DNS server, which will overwhelm other DNS operations and make them time out as well -- including legitimate messages to legitimate domains!
There are three ways to work around this problem:
About the author: Serdar Yegulalp is editor of the Windows Power Users Newsletter.
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When you turn on recipient filtering's "Filter recipie
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nts who are not in the directory," you also need to safeguard against a directory harvest attack once this is enabled. It is imperative to set the tar pit feature introduced prior to Exchange's 2003 SP1 (via hotfix) to any outside facing server. Configure the registry to use the tar pit feature. (Warning: If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.)
To enable the tar pit feature, you must add the TarpitTime registry entry to the registry and then configure the delay time value. To do this, follow these steps:
(Note: If the TarpitTime registry entry does not exist, Exchange Server behaves as if the value of this registry entry were set to 0. When the TarpitTime registry entry has a value of 0, there is no delay when the SMTP address verification responses are sent.)
—Poomba1
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Related information from SearchExchange.com:
Learning Guide: Spam
Tip: Dealing with undeliverable messages in Exchange Server
Tip: Stay above the SMTP queue floods
15 tips in 15 minutes: Managing recipients and distribution lists
Reference Center: Exchange Server and DNS tips and resources
Reference Center: Exchange NDR tips and resources
