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  • Anti-spam/anti-virus e-mail screening

    Written by wan@trglib.gov.my

    Mon, 08 Sep 2008

    Anti-spam/anti-virus e-mail screening

    The following applies for all incoming messages, whether personal or coming through a mailing list.
    Every message goes through  filters; each is apt to reject a message if it is detected as undesirable according to specific criteria.Below is a recap of the checks performed by each filter in its order of application: if a message is rejected by any one filter it does not each the following one(s).If you run into a problem, please remember to transmit the full message, including (especially!) all the headers, so we can diagnose it.

    At this stage, messages are rejected when:

    • they come from senders who grossly violate e-mail emission standards (which is only the case for spammers);
    • they contain an attached file with an uncertain extension name (list provided by Microsoft, including in particular .bat, .exe, .pif, .reg. and .url)
    • they contain an attached file and are addressed to a mailing list
    • they attempt to be interpreted as a legitimate message sent from Outlook
    • they contain more than 15 characters with accents (not French) and a variety of symbols in the Subject line (Asian and other spam)
    • they are part of a number of other cases specific to such or such spam or worm.
    If a message is rejected following one of these checks, it is not accepted by the system at all

    Spamassassin

    The third filter uses the "spamassassin" program, which performs numerous checks on the message, in both its form and its content.It checks, for instance:

    • that the message was not sent from an IP address known to be that of a spammer
    • that the message does not contain certain words/sentences/addresses from Web sites known to be included in spam
    • that the message is not similar to a message already considered as spam by other servers
    • if the message is purely HTML (used more often by spammers)
    • etc.

    Spamassassin then gives a score, which is a weighted average of hundreds of tests performed. A score higher than 5 is very likely to be spam, but there can always be mistakes.

    Spamassassin rejects a message with a score of 15 or more (and the sender is informed), but under that score, it allows messages to get through and adds the headers: X-Spam-Status, X-Spam-Level, X-Spam-Checker-Version, X-Spam-Flag, X-Spam-Report. That way, you can set up your e-mail software and/or webmail systems to filter incoming messages containing "X-Spam-Flag: YES" into a special file, or even to destroy them. This, however, would be your own responsibility, which is why the server only adds these headers and does not destroy the messages (unless the score is 15 or more, at which point there is no doubt that it is spam).

    Anti-virus

    This filter puts the message, the attached files and possible archive contents in attached files through an anti-virus program (clamav) that is updated on an hourly basis.If a virus is detected, the message is rejected and the sender is informed (for the very unlikely case where the sender is legitimate).Regardless of this filtering, we recommended that you always have an updated anti-virus program on your computer.

     

    Harap Maklum .

     

    Unit Teknologi Maklumat PPAT