How To Read 950 E-mail Messages Before Lunch

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

I have heard it said that electronic mail is the biggest reason that people first go to the Internet, and that Usenet news is why they stay. My question is, what is the single biggest reason that people leave the Internet? The answer is junk mail! Well, maybe they haven't left. It just looks that way because they never answer e-mail. It's easy to understand why in a public forum that spans the globe and includes tens of millions of members, people might be receiving more mail than they can read.

There are thousands of opportunities to subscribe to mailing lists, and interneters can quickly overfill their plates. I know, I've done it. In this article I'll discuss a powerful set of tools that allow you to get control of your in-box and reduce your chances of heart trouble related to starting your mail reader. These tools are called “E-mail filters”. E-mail filters are programs that sort mail based on a your directions. For instance; all mail from my brother should be moved from my in-box mail folder to a folder labeled “Frank”. Filters work by processing mail, after the system delivers the mail to you, but before you actually read it.

What do I mean by that?

How an e-mail Transaction Works

Let's start at the point that mail for you is delivered. When mail is delivered to your computer on the Internet, it arrives via a mail system which is using Simple Mail Transfer Protocol (SMTP). Although there is probably a different e-mail daemon for every week of the year, sendmail is by far the most popular. For the rest of this article when I speak of the “mail system” I'll just call it sendmail, though most of what I say applies with smail, MMDF, and others. You can ignore the differences for the purposes of this article.

So, to continue, “sendmail” takes the e-mail and then decides whether the e-mail needs to be sent directly on to another system or delivered locally. If sendmail decides that the mail should be delivered locally, it goes through a short list of actions. First it looks to see if there is a .forward file in the user's home directory. If there is no .forward file, sendmail writes the mail to the user's system-wide mail file.

If the user does have a .forward file, sendmail reads the file. If the file contains an e-mail address, sendmail fowards the message to that address. If the forward file contains a pipe | character, sendmail runs the specified program, sending it the mail message, letting the program deliver the mail. This last case is how e-mail filters work. The sendmail daemon “delivers” the mail to your filter, and the filter delivers it (or doesn't deliver it, if you prefer) to the folders, following your set of rules. If you use the elm filter program, your .forward file might look like this:

"| /usr/local/bin/filter"

sendmail would deliver all your mail to the program called filter.

What would happen if filter was not there, or otherwise broken? We can guard against failure, by providing an alternative for sendmail.

"| /usr/local/bin/filter || exit 75 "

In this example, if the filter fails, the delivery to the user's .forward file will exit with an error number 75. This forces sendmail to back off the .forward file, and try again later instead. The || exit 75 only protects against catastrophe, not bad choices. If you do not correctly configure your filter, it may lose mail, but would not “fail” from the perspective of sendmail. The exit 75 would not help you to get the mail back.

The most common place the exit 75 can help you is when your home directory runs out of space. Most filters will gracefully fail, allowing your mail to be delivered to the system mailbox instead. This is especially helpful on systems that have disk quotas on the home directories, because the mail spool generally does not have user quotas.

Choosing a Mail Filter

There are at least four popular mail filters available: Procmail, Elm-filter, Mailagent, and MH's slocal. Procmail is a robust general purpose mail filter. By design, it is small, easy to install, and dependable. Elm is a user mail program for reading and sending e-mail. The Elm-filter is a separate program that comes with the Elm package, and can be used with or without the rest of Elm. Slocal is the mail agent that comes with the Rand corporation's Mail Handler (MH). You might be buying the Cadillac for the cigarette lighter if you install MH just to use slocal, though. Unfortunately, Slocal does not support regular expressions (see msort sidebar for a possible solution). In contrast to Slocal, the mail filtering package called “Mailagent” supports a very rich regular expression syntax. Unlike the other filters which are written in C, mailagent is written primarily in Perl, and uses Perl's powerful regular expressions.

Procmail can write mail to “mbox” style mail files, as well as MH style mail directories. Slocal can write to “mbox” style mail files, as well as its native MH style “folders” (directories). In general, mail agents can be used interchangably with many different e-mail readers.

I use procmail to filter my mail, MH for my mail package, and “Exmh” as an X-based frontend to MH. (Exmh is written in Tcl/Tk, and is possibly the best way to do e-mail. [I agree!---ED])

If you are lucky, one or more of the e-mail filters described here may already be installed on your system.