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| Helpful Hints | March 2002 |
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Anti-Relaying with Mail-Servers While POP3 and IMAP4-Servers are responsible for mailbox-access, SMTP servers take on the task of forwarding and distributing electronic messages, which is also called Mail Relay. Not only users and mailbox-owners try to keep their accounts free of Spam; there are also server-side ways to protect yourself from Spam. The magic word is Anti-Relaying. SMTP servers fulfill a central function; they receive incoming messages, check the domain name system, and forward messages to another SMTP server, which finally places them in a recipient's mailbox. Mail-Servers usually get into trouble if they are confronted with Spam. It not only costs valuable transfer volume, but also means higher maintenance and operating costs. The administrator sees himself placed in an even greater dilemma: From the recipient's perspective it looks like the "own" server delivers all the unwanted mail. Several Server-side protective mechanisms proofed themselves effective. Some are already standard in current Mail-Servers. The simplest solution to reject incoming messages and stop their forward, are filter mechanisms, which give the administrator permission to exclude IP addresses or whole Domains. The way Anti-Spam tools function are almost identical on both ends (Client-side and Server-side). Server-side mechanisms are merely a bit "more intelligent" than Client-side ones. Methods which check messages for an MX entry and only forward them if they were received from local registered users, or reject them, if the number of recipients exceeds a defined limit, are far more reliable. In connection with messages to the system administrator, who then determines, what may be forwarded and what not, such functions offer optimal protection. With Server-side Anti-Spam functions, which can be found on Mail-Servers (e.g. Mercur), Spam content is deleted, which means there is no way that unwanted mails find other ways to its target audience. So much for the theory; in practice however different problems can occur: Only newer SMTP servers offer Anti-Relaying functions. Unfortunately there are far more outdated systems in operation, which can be found in particular in the "Internet developing countries". Spammers therefore prefer Mail-Servers in countries such as or Pakistan, Sri Lanka or Eastern European countries. Nowadays there are special solutions available, which extend existing SMTP Server-functions. An example would be the Mail-Server Plug-in "Mailshield". The system uses over fifty different techniques for Mail recognition, which can be used for the message-analysis. Filter functions can easily exclude MMF or other definable content from forwarding. To create even more effective filters against unwanted mail, Mailshield contains a so-called "Set of Rules", which can be adjusted to almost any needs. Mailshield can be used with Sendmail, Exchange, Notes/Domino, Post Office and Mdaemon. The program is available for Windows NT, Windows 95/98, Sun Solaris and AIX. Further Unix versions are in development. A demo version is available on the Mailshield Website. Mailshield costs $995.00 US per server. A software-hardware combination solution is the BSDI Mailfilter. The device has the size of an Ethernet-Hub and is plugged before the Mail-Server and deletes over 95% of all Spam-mails in a matter of minutes. After the system is online it automatically connects to the manufacturer's homepage and downloads the latest software-version and all the current Spam-databases. The system is available in three models: Model 10 costs $2400 US, Model 100 up to $5000 US and Model 1000 almost $9000 US. In addition there are annual service fees for updating and maintaining the system of the Spam-databases at a price of $1400 US, $4900 US and $12,500 US. Philipp Eirich |
