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Use an appliance to reduce spam's impact on Exchange servers

You may want to consider offloading spam filtering to an antispam appliance to avoid performance issues.

Experts disagree about just how much spam is out there, but one thing they do agree on is that the majority of e-mail is spam. Current estimates indicate that anywhere from 60% to 95% of inbound e-mail is spam. In my own Exchange organization, over 90% of the mail that my Exchange server receives is spam. That means that only one out of every 10 messages is legitimate.

Spam places a tremendous burden on Exchange servers just because of its sheer volume. There are some basic steps that you can take to reduce the strain that spam puts on your servers. For example, a lot of companies disable non-delivery reports. That way, servers don't respond every time someone attempts to send a spam to a mailbox that doesn't exist. Disabling mail relay is also essential in combating spam. But these techniques alone don't really get to the root of the problem.


Once your Exchange servers receive spam, the message must be scrutinized by your servers' antivirus and antispam software prior to ultimately being either deleted or delivered to users' inboxes. Even if you have incredibly good antispam software that filters out all spam, the simple act of filtering places a huge performance hit on your servers.

So how do you get around the performance impact caused by processing spam? It's simple -- don't allow your servers to process spam. If your Exchange servers are overburdened, why not offload as many tasks as possible? You can't free up your servers from having to run your Windows operating system or Exchange application, and it isn't safe to not run antivirus software, but you can offload spam filtering.

One way of doing this is to invest in an antispam appliance that sits between your Internet connection and your Exchange organization. When messages are sent to your domain, they will pass through the antispam appliance, where they will be filtered prior to being delivered to the Exchange servers.

How much of a difference will this make in Exchange server's performance? A lot of it depends on the volume of spam that your organization receives and how effective your antispam appliance is at catching spam. For the sake of argument though, let's say that 80% of the mail sent to your domain is spam and that you bought a cheap antispam appliance that only manages to catch about half of it. Even with such a mediocre appliance, you have still lightened the volume of mail that your Exchange server has to deal with by 40%! Furthermore, you have also relieved Exchange of the task of having to filter spam itself, so the performance gain will actually be much greater than it would be if you were only reducing the volume of mail.

About the author
Brien M. Posey, MCSE, is a Microsoft Most Valuable Professional for his work with Windows 2000 Server and IIS. Brien has served as CIO for a nationwide chain of hospitals and was once in charge of IT security for Fort Knox. As a freelance technical writer he has written for Microsoft, TechTarget, CNET, ZDNet, MSD2D, Relevant Technologies and other technology companies. You can visit Brien's personal Web site at

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This was last published in March 2005

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