Using foo+bar@ email addresses with Postfix and procmail

Using foo+bar@ email addresses with Postfix and procmail

Mail addresses can be expressed as localpart@example.org. The
key point of this article is that the localpart is indeed local.
This provides a wide array of possibilities. I will explore just one of those possibilities.

Address extensions have been around for some time. They are a conceptual entity and are
not actually defined in any RFC.
Convention dictates what an address extension is and how it is used. Let us use
foo+bar@example.org as our primary example. The user name
in this example is foo. The extension part is bar and
can be used for variety of purposes, but the most common usage is folder delivery.
It is important to note that the delimiter can be anything you want. That is because
delivery is a local matter. We can do whatever we want. Common delimiters include ‘+’ and ‘-‘.
I have chosen to use ‘+’. Postfix allows you to set
this value using the recipient_delimiter setting.

For reference, the following may be useful:

• RFC 822 – STANDARD FOR THE FORMAT OF ARPA INTERNET TEXT MESSAGES
• RFC 2822 – Internet Message Format

Yeah, well, why bother?

Why am I doing this? Why bother with address extensions? The answer is simple: because it makes life
easier.

In this particular case, I am setting up a new development box for
FreshPorts and FreshSource.
This box is much faster than the existing environment. It will contain a development, a test, and a beta
environment. I will want cvs-all messages
going to each of those databases. I do this by sending the messages to dan+dev, dan+test, and dan+beta
respectively^[1]. I use procmail to process the incoming mail. This is
very useful as it allows me to process the mail into the appropriate FreshPorts database. Without
this ability, I would resort to different users for each database. Having just one user (me!) makes
this all very easy and simple.

^[1] – no, I don’t have three subscriptions to the cvs-all mailing list. I have one. I
use local aliases to copy that single message to three local addresses.

What’s in it for me?

You might be asking that question now. Let me give you an example. Let’s guess that you are subscribed to
three different mailing lists. It’s nice to keep all the messages for a particular list in a separate
folder. There are many different ways to do this. I will not list them. Let’s assume all the
mailing lists have nothing in common and therefore you need three different entries such as these:


:0
* ^Sender: owner-cvs-all@FreeBSD.ORG
lists/cvs-all

:0
* ^Sender: Sender: procmail-bounces@lists.RWTH-Aachen.DE
lists/procmail

:0
* ^Sender: pgsql-advocacy-owner@postgresql.org
lists/postgresql-advocacy


Worse still, there is nothing in the mailing list email messages which is easy to identify. Rather
than check To:, From:, and possible CC:, you can avoid all this by adding your very own key; your
subscription address. Let’s assume I subscribe to the above lists using the following email addresses:

• dan+cvs-all@example.org
• dan+procmail@example.org
• dan+postgresql-advocacy@example.org

If you did that, then your procmail recipe becomes:


:0
$ARG


where ARG is the address extension.

But how do you get that extension? Postfix to the rescue!

Address extensions with Postfix

The Postfix MTA supplies many environment variables
when local delivery is being performed. man 8 local
contains the information you need. Specifically, this extract from the EXTERNAL COMMAND DELIVERY section:


EXTENSION
       The optional recipient address extension.


This variable is available to you within your ~/.forward script, which is the
normal method for invoking procmail. Here is what I’m using:


"|exec /usr/local/bin/procmail -t -a ${EXTENSION}"


Read man procmail for more information.
on these options.

• -t instructs procmail to fail softly.
• -a specifies an argument (in this case, the address extension).

In case anyone is wondering, I looked at using Procmail
for system-wide local delivery
but decided against it. Although I am the only user on this system, I didn’t want to invoke procmail for
every single email delivery.

See also the man page section titled ADDRESS EXTENSION where it documents that
~name/.forward+foo is tried first, by default. This setting can be changed
via the forward_path setting (see sample-local.cf for
an example).

The new procmail recipe

Here is how to use the address extension (as supplied by Postfix) within procmail. I found this gem
in the procmail tips.


ARG = $1

:0
* ARG ?? ^^cvs-all^^|^^procmail|^^postgresql-advocacy^^
$ARG


The address extension is passed in as $1 by the .forward
script. You can add more parameters to .forward as needed. The first is always
$1, the second will be $2, etc. You cannot access this
variable directly and you must assign it to a variable.

Passing the extension along

Sometimes you need to forward an email and include the extension you were given. This recipe does that:


ARG = $1

:0
* ARG ?? ^^^^
    !dan@things.example.org

:0
    !dan+$ARG@things.example.org


^^^^ represents an empty string. In this case, we do not include an extension.

Other references

I found Procmail Quick Reference Guide
useful when devising my recipes.

The last word

My thanks to those on the procmail and Postfix mailing lists for pointing me in the right direction.