E-mail with attached files appears empty to DCC

Vernon Schryver vjs@calcite.rhyolite.com
Sun Mar 28 01:21:52 UTC 2004

> From: Gary Mills 

> We've had a few complaints from professors that some of their students
> are unable to e-mail assignments to them because of our DCC bulk mail
> filter.  These students use one of the free e-mail services, such as
> Hotmail or Yahoo.  They simply attach a Word file to an e-mail message
> with an empty body, and send it, expecting it to work.
> Of course, the body of the message is really full of invisible multi-
> part MIME and HTML goo, with an advertizing footer, making it appear
> identical to many other empty messages from these services.  DCC rejects
> it as bulk mail.
> Is there anything that can be done with checksums so that DCC can detect
> that such e-mail actually contains unique content, in the form of an
> attachment?

If the attachments were in cleartext instead of encyphered in the one
of the proprietary formats of a convicted monopolist whose purpose in
such mail is to extend and protect the monopolist's empire, the fuzzy
checksums would recognize them as distinct, if they are distinct.  It
also wouldn't hurt if those professors would set good examples for
their students by using services that are not significant contributors
to the spam problem by hosting throw-away drop boxes.

In other words, the DCC fuzzy checksums do look in attached multipart
MIME and HTML goo for text.  They have as much trouble finding it in
Word documents as `vi`, `emacs`, and `strings`.

Besides, are you sure those messages are not simply "bulk"?  Are you
sure those professors are not sending bit-for-bit identical copies of
single messages to many dozens or even hundreds of students?  If so,
an answer might be for the professors to use reputable, white-listable
mail systems for their bulk mailings, such as your systems.

There's yet another possibility that turned up at another institution
recently.  Are you sure those professors are not sending from IP
addresses that are in blacklists of "dialup" or "dynamic" IP addresses?

Vernon Schryver    vjs@rhyolite.com

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