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By John Junod If you own and operate a Simple Mail Transfer Protocol mail server, chances are you have been a victim of spam mail - perhaps without even being aware of it. Spam, as the e-mail community recognizes the term, is unsolicited commercial e-mail: the electronic version of all that colorful junk mail the U.S. Postal Service delivers every day. The burden of delivering postal junk mail is on the sender: They pay for the printing, handling and postage. You simply have to weed it out. With spam, however, it costs the sender next to nothing to create and send large numbers of messages, but it can cost the recipients and the handlers plenty. For example, users lose time and money in connectivity costs, and Internet service providers and other owners of commercial e-mail servers lose control of resources they need for their businesses. The spammer, however, gets a free ride via SMTP. SMTP was created in the early days of the Internet to provide a standard communications protocol for data transfer among a multitude of different computer systems. SMTP was designed not to question the content of a message, the nature of its origin or its ultimate destination, but to transfer a string of packets from wherever it was coming from to wherever it was programmed to go. This allows different computers to share the same electronic highway, where the traffic laws are common enough for even incompatible systems to mutually understand each other. While the required openness of SMTP is a blessing, it also is its curse. It provides the purveyors of unsolicited commercial e-mail with free reign to relay thousands of promotional messages off any SMTP mail server in the world - sometimes tying up the servers for unacceptable periods of time and degrading the service for legitimate users. Usually, it's not until you have noticed an unexpected load on your mail server, which slows your network to a crawl, that you realize your mail system has just been hijacked by spam. Your SMTP server, without questioning the motives or implications of the request, dutifully obeyed the spammers' instructions to deliver 500,000 advertising messages to unwilling recipients. Or maybe you find out youve been spammed after 100,000 of those spam recipients flame you because the spammer made it look like the mail originated from your system. Fighting backToday, there are several methods you can employ to prevent spam from infiltrating your internal e-mail community and hijacking your server. The simplest way to avoid receiving spam is to deploy a filter that blocks e-mail from any known source of unsolicited commercial e-mail. The problem is knowing all the sources and denying them access without cutting off mail you really want to receive.There are two ways of preventing unauthorized use of your SMTP mail server as a relay for spam. The first is to deploy two servers: one, external to a firewall, for accepting mail intended for your users, and the other inside a firewall, dedicated to sending mail only from your users to the outside world. Under this scenario, the SMTP server on the outside is not configured to relay anything outside of your domain, hence all mail not destined for recipients inside your firewall will be rejected. The internal SMTP server is fully enabled for relay, so the only mail it will pass on is legitimate mail from your user community. The cost of securityThis can be an expensive proposition once you add up the cost of the firewall, hardware and software for two servers. This may work for large companies with deep pockets, but for most cost-conscious operations a second solution is called for: a single SMTP mail server with built-in relay-prevention features.Certain SMTP mail servers on the market allow you to deny the relay of mail from any server other than the ones you have specifically approved. This feature was first included in SendMail 8.8 for Unix and is now available in two mail servers for Windows NT - IMail Server 4.0 from Ipswitch, Inc. and Post.Office 3.1 from Software.com, Inc. Other mail servers for Windows NT, including Lotus Development Corp. Notes and Microsoft Corp. Exchange, offer the ability to block e-mail from identified spammers, but they currently do not allow the server to refuse unwanted relays. Ideally, a server with a full complement of antispamming features should allow you to not only block undesirable sources of junk e-mail, but also protect yourself from unwanted relays. The technology for controlling the rising tide of unsolicited commercial e-mail and preventing the unauthorized use of Internet resources is still evolving. The openness of SMTP will always make us vulnerable to abuse because we need a system of common relays throughout the Internet to guarantee delivery of e-mail to every location on the planet, which is the whole idea.
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