The Annoyance Filter.pdf - Fourmilab

§6 ANNOYANCE-FILTER INTEGRATING WITH PROCMAIL 11
The action line which pipes the message to annoyance−filter is continued onto a second line here in order
to fit on the page. Procmail permits continuations of this form, but will equally accept the command all on
one line with the backslash removed.
6.3.2. Routing by annoyance−filter classification
Once the message has been filtered by annoyance−filter, subsequent rules can test for its classification
and route the message accordingly. The following rules dispatch messages it classifies as junk to a junk
folder used by the blacklist, while messages judged to be legitimate mail and those with an intermediate
probability are sent to the user’s mailbox. (With the default settings, annoyance−filter will always classify
a message as mail or junk, but if the −−threshjunk and −−threshmail settings are changed to as to create
a gap between them, intermediate classification can occur.) Actually, the latter two recipes could be omitted
since any message which fails to trigger any Procmail rule is sent to the user’s mailbox by default. The
variable $ORGMAIL is defined by Procmail as the user’s mailbox; using it avoids using the specific path name
which is dependent on the user name and mail system configuration.
:0 H:
* ^X-Annoyance-Filter-Classification: Junk
junk
:0 H:
* ^X-Annoyance-Filter-Classification: Mail
$ORGMAIL
:0 H:
* ^X-Annoyance-Filter-Classification: Indeterminate
$ORGMAIL
Even if you set the mail and junk probabilities so that messages can be classified as “Indeterminate”,
you’re unlikely to see many so categorised—as long as the collections of mail and junk you used to train
annoyance−filter are sufficiently large and representative, the vast majority of messages will usually be
scored near the extremes of probability. If you’re seeing a lot of Indeterminate messages, you should sort
them manually, add them to the appropriate collection, and re-train annoyance−filter.
If you have other Procmail recipes for handling specific categories of mail, you would normally place
the annoyance−filter related recipes after them, at the very end of the procmailrc file. That way
annoyance−filter’s evalution is used as the final guardian at the gate before a message is delivered to
your mailbox.
6.3.3. Other useful .procmailrc rules
The following subsections have nothing at all to do with annoyance−filter, really. You can set up a
.procmailrc file based exclusively on annoyance−filter classifications as described above. Still, in many
cases a few Procmail rules are worthwile in addition to annoyance−filter filtering. Here are some frequently
used categories. You would normally place these rules before the annoyance−filter rules discussed in section
3.2.
6.3.3.1. Whitelist
Most people have a short list of folks with whom they correspond regularly. It’s embarrassing if the content
of a message from one of them is mistakenly identified as junk mail. To prevent this, define a “whitelist” as
the first rule in your Procmail configuration after the filter command; messages which match its patterns
avoid further scrutiny and are delivered directly to your mailbox. You should generally include your own
address in the whitelist, as well as addresses of administrative accounts on machines you’re responsible for,
but be careful: junk mailers increasingly use sender addresses such as root to exploit whitelists. Here’s
user blohard’s whitelist definition. Multiple Procmail rules are normally combined with a logical AND (∧)
operation. Since the whitelist requires an OR (∨) operation, we manufacture one by a trivial application of
Procmail’s weighted scoring facilities. Procmail patterns are regular expressions identical to those used by
egrep, so metacharacters such as “.” must be quoted to be treated literally in patterns.