How To Read 950 E-mail Messages Before Lunch

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A discussion of the use of e-mail filters on Unix computers that use Sendmail-like mail systems.
More Advanced Rules

PGP key handling has a special need. People who use Phil Zimmerman's PGP program to send encrypted e-mail need to exchange “public keys” to communicate securely. A common convention is to put the public portion of the PGP key in the .plan file. Anyone who wants the key can then just finger you. This is great, unless your system does not support finger, or your account is behind a firewall. The next best thing might be some sort of automatic mail response. Using procmail, you can filter for a special phrase in the subject line, like this:

:0 h c
* !^FROM_DAEMON
* ^Subject.*SEND-PGP-KEY
| (formail -r -A"Precedence: junk";\
   cat ~/.plan ) | $SENDMAIL -t

This procmail recipe first checks to see if the mail comes from the mail daemon, to prevent e-mail loops, or “ringing”. Then if the subject includes the special phrase SEND-PGP-KEY, procmail invokes formail which automatically constructs a reply to the sender. The e-mail that is sent back includes the contents of the .plan file. If you keep your PGP public key in this file, anybody can request a copy of your key, even if they can't finger you. The equivalent using Elm-filter would look like:

if ( subject contains "SEND-PGP-KEY" ) then
  execute "cat ~/.plan | mail -s \"RE: %s\" %r"

Elm's filter uses a macro %s to represent the original subject of the message, and %r to represent the return address.

Mail agents are really great if you have special e-mail needs. I carry an alphanumeric pager. I use procmail to watch for mail that has the magic word PAGEJAY in the subject line. If procmail sees the magic word two things happen. First, a copy of the e-mail is forwarded to my e-mail->pager gateway at page-jay@pager.fork.com. When the forwarded message gets through the gateway, it will be broadcast to my pager. (The folks in my office call this “Belt-Mail”, or if the pager is set to vibrate, it's an e-mail massage.) Second, a copy of the e-mail is stored in a local folder called “pages”. Here is what the procmail recipe looks like:

:0 c
* ^Subject.*PAGEJAY
! page-jay@pager.fork.com
:0
* ^Subject.*PAGEJAY
pages/.

The first recipe uses the c flag. This tells procmail that even if there is a match, the matching should continue past. In the next recipe, the belt-mail will get filed, and the matching of this e-mail will end. The equivalent in Elm's-filter rules would be:

if ( subject contains "PAGEJAY") then
      forward "page-jay@pager.fork.com"
      save "~Mail/pages"
endif

E-mail filters/agents are powerful tools that enable people who get lots of e-mail to communicate effectively. From the mundane but important task of sorting e-mail, to the complex task of responding automatically to e-mail requests, filters can deliver. Everyone involved benefits from the use of filters. You like it because your mail is always sorted the same way (Where did I put that CERT advisory?). The people who send e-mail to you like it because you start to respond faster by e-mail than via Canadian “First Class”. The only people who don't like it are the direct-marketing-spamers who keep trying to send you ads for “Investment opportunities”. It's OK if they get mad, because you don't ever see their mail since you installed the filter.

Any time spent learning how to configure and use the filters is won back many-fold in time-savings later.

For more information about e-mail filters, see sidebar.

Jay Allen (jay@fork.com) got his BS in chemistry from Portland State University in 1991. He is currently a lead Unix Systems Engineer at Blue Cross/Blue Shield of Oregon. Among other things, he is interested in the use of cryptography for commerce on the Internet, and secure private networks.

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