Postfix: altering the subject line of outgoing messages

Motivation: I’m changing my ticketing system so that messages with a friendlier subject line. Instead of ‘Subject: [SNAFU #1] summary’, I’ll change it to use ‘Subject: [HELPDESK #1] summary’.

Another colleague suggested we use procmail. As unixgeeks.org says:

Is procmail right for me?
Procmail is a serious unix hack. On the other hand, it does a pretty good job.

(http://unixgeeks.org/security/newbie/unix/procmail/procmail.html)

I think we can do better than a serious Unix hack. I think Postfix can handle this.

How to do it?

First, I created a test environment: two Debian 9 virtual machines:

• foo.osric.net
• bar.osric.net

I’m not going to add DNS entries, though, so I’ll be relying on their IP addresses on my internal network:

• 192.168.0.18
• 192.168.0.19

I installed postfix and mutt on each:

# apt-get install postfix mutt

As an initial test, I tried to send a message from one machine to the other:

# sendmail root@192.168.0.18 < test.eml

But this resulted in an error from the mailer-daemon:

<root@192.168.0.18>: bad address syntax

The solution? Enclose the IP address in square brackets:

# sendmail root@[192.168.0.18] < test.eml

(Thanks to Is there a way to send an email to an IP address directly?, which I believe is the first time I have ever seen anything useful on the Quora web site.)

The two test systems can send messages to each other. Great!

Now, to change the subject line. A ServerFault post, postfix header_checks to rename email subject, looked like exactly what I was looking for (and a similar scenario motivated that question).

Following the answer there, I added the following to /etc/postfix/main.cf:

header_checks = pcre:/etc/postfix/header_checks.pcre

And I created /etc/postfix/header_checks.pcre, containing the following:

/^Subject: \[SNAFU #(\d+)\](.*)$/    REPLACE Subject: [HELPDESK #$1]$2

Don’t forget to reload Postfix, required by just about any Postfix change:

# postfix reload

I sent a test message from bar to foo, and an error message arrived in root’s mailbox on foo:

Subject: Postfix SMTP server: errors from bar.osric.net[192.168.0.19]

I checked the logs at /var/log/mail.log and found the following:

Jul  8 16:47:29 foo postfix/cleanup[10927]: error: unsupported dictionary type: pcre

I reproduced this problem using postmap:

# postmap -q 'Subject: [SNAFU #1]' pcre:/etc/postfix/header_checks.pcre
postmap: fatal: unsupported dictionary type: pcre

I changed the filename (not strictly necessary, but good for my sanity) and tried regexp instead:

# postmap -q 'Subject: [SNAFU #1]' regexp:/etc/postfix/header_checks.regexp

This produced no output, no error message, and a return code of 1. Why didn’t it match?

It turned out I needed to modify the regular expression, since \d is a Perl-ism:

/^Subject: \[SNAFU #([0-9]+)\](.*)$/    REPLACE Subject: [HELPDESK #$1]$2

(Using [[:digit:]] instead of [0-9] would also work.)

I ran a couple successful tests using postmap:

# postmap -q 'Subject: [SNAFU #1]' regexp:/etc/postfix/header_checks.regexp
REPLACE Subject: [HELPDESK #1]

~# postmap -q 'Subject: [SNAFU #1] more text' regexp:/etc/postfix/header_checks.regexp
REPLACE Subject: [HELPDESK #1] more text

Now to test an actual e-mail message.

First, I updated /etc/postfix/main.cf to point to the updated filename with the corresponding dictionary type:

header_checks = regexp:/etc/postfix/header_checks.regexp

Don’t forget:

# postfix reload

I tested it from bar.osric.net (192.168.0.19), using the following file, test.eml:

Subject: [SNAFU #1] test message

This is a test.

# sendmail root@[192.168.0.18] < test.eml

It appeared in root’s inbox on foo.osric.net (192.168.0.18) with the updated subject as expected:

Date: Sun,  8 Jul 2018 17:03:56 -0500 (CDT)
From: root <root@bar.osric.net>
Subject: [HELPDESK #1] test message

This is a test.

I sent another message with a different subject line just to confirm it was unchanged by the header_check.

Other useful resources:

• Postfix documentation: header_checks
• Postfix documentation: postmap
• Postfix documenation: regexp table