How to Add Different Disclaimers using alterMIME and Postfix based on Domain

As you may have gathered from my previous blogs, I like Postfix, one thing I love about it is how modular it is.  This became especially evident today when I was asked to make sure that all emails go out with the appropiate disclaimer on it.  This company has one MS Exchange mail server which services a hand full of different companies’ domains.

It transpires that adding the pretty signature via a pretty Outlook stationary item is not an option, with the mix of ActiveSync enbaled devices among staff (ie iPhone & HTC), people using Webmail at home signatures were not always getting applied neatly via Outlook.  So we had to find a workaround to make sure the stationary gets attached.

My first thought was on the Exchange itself, I knew that it was possible to add stationary via the SMTP sink as per this Microsoft Article, but this didn’t strike me as being a particularly elegant solution on an already heavily utilised box, plus MSExchange.org indicated that it may increase the load on the server.

In the end we decided upon doing it at al MTA level on our existing Ubuntu Postfix MTA and then use alterMIME to add the necessary text.  We found two excellent resources that helped a great deal an article by Falko and another on the Zimbra Wiki.

We present here a combination of the steps we took with the direction from the two posts above and this would not have been possible without those guys sharing for the greater community.

Via your Ubuntu shell install alterMIME

apt-get install altermime

Next we need to create a user for alterMIME and set it up with a directory.

useradd -r -c “Postfix Filters” -d /var/spool/filter filter
mkdir /var/spool/filter
chown filter:filter /var/spool/filter
chmod 750 /var/spool/filter

Next we will install the default script that does the work.

cp /usr/share/doc/altermime/examples/postfix_filter.sh /etc/postfix/disclaimer

As discussed in the above articles alterMime requires some tweeking if you don’t want it to apply a standard disclaimer to all inbound and outbound email.  We start this by creating different lists who will use different disclaimers.  We’ll worry about populating later on.

touch /etc/postfix/disclaimer_addresses_companya
touch /etc/postfix/disclaimer_addresses_companyb
touch /etc/postfix/disclaimer_addresses_companyc
touch /etc/postfix/disclaimer_addresses_companyd

While we’re at it, we may as well copy across the default disclaimers that alterMime gives us (we can change them later with our specifics).

cp /usr/share/doc/altermime/examples/disclaimer.txt /etc/postfix/disclaimer_companya.txt
cp /usr/share/doc/altermime/examples/disclaimer.txt /etc/postfix/disclaimer_companyb.txt
cp /usr/share/doc/altermime/examples/disclaimer.txt /etc/postfix/disclaimer_companyc.txt
cp /usr/share/doc/altermime/examples/disclaimer.txt /etc/postfix/disclaimer_companyd.txt

Next we want to edit the configuartion file to take notice of the different email domains.

Lets backup the config file since we’re going to mess with it and it’s always nice to have a good copy of it.

cp /etc/postfix/disclaimer /etc/postfix/disclaimer.orig

Lets open the default config.

nano /etc/postfix/disclaimer

Delete all that in there already and replace with the following, txt version here.

#!/bin/sh
# Localize these.
INSPECT_DIR=/var/spool/filter
SENDMAIL=/usr/sbin/sendmail

# Define our company address lists
disclaimer_addresses_companya=/etc/postfix/disclaimer_addresses_companya
disclaimer_addresses_companyb=/etc/postfix/disclaimer_addresses_companyb
disclaimer_addresses_companyc=/etc/postfix/disclaimer_addresses_companyc
disclaimer_addresses_companyd=/etc/postfix/disclaimer_addresses_companyd

# Exit codes from <sysexits.h>
EX_TEMPFAIL=75
EX_UNAVAILABLE=69

# Clean up when done or when aborting.
trap “rm -f in.$$” 0 1 2 3 15

# Start processing.
cd $INSPECT_DIR || { echo $INSPECT_DIR does not exist; exit
$EX_TEMPFAIL; }

cat >in.$$ || { echo Cannot save mail to file; exit $EX_TEMPFAIL; }

# Obtain From address domain
from_address=`grep -m 1 “From:” in.$$ | cut -f 2 -d ‘@’ | cut -d “>” -f 1`

# If the from address matches companya then apply the appropiate disclaimed
if [ `grep -wi ^${from_address}$ ${disclaimer_addresses_companya}` ]; then
/usr/bin/altermime –input=in.$$
–disclaimer-html=/etc/postfix/disclaimer_companya.txt ||
{ echo Message content rejected; exit $EX_UNAVAILABLE; }

# If the from address matches companyb then apply the appropiate disclaimed
if [ `grep -wi ^${from_address}$ ${disclaimer_addresses_companyb}` ]; then
/usr/bin/altermime –input=in.$$
–disclaimer-html=/etc/postfix/disclaimer_companyb.txt ||
{ echo Message content rejected; exit $EX_UNAVAILABLE; }

# If the from address matches companyc then apply the appropiate disclaimed
if [ `grep -wi ^${from_address}$ ${disclaimer_addresses_companyc}` ]; then
/usr/bin/altermime –input=in.$$
–disclaimer-html=/etc/postfix/disclaimer_companyc.txt ||
{ echo Message content rejected; exit $EX_UNAVAILABLE; }

# If the from address matches companyd then apply the appropiate disclaimed
if [ `grep -wi ^${from_address}$ ${disclaimer_addresses_companyd}` ]; then
/usr/bin/altermime –input=in.$$
–disclaimer-html=/etc/postfix/disclaimer_companyd.txt ||
{ echo Message content rejected; exit $EX_UNAVAILABLE; }

fi

# Send the email on as per the Postfix stack
$SENDMAIL “$@” <in.$$

exit $?

Now we need to make the file executable.

chgrp filter /etc/postfix/disclaimer
chmod 750 /etc/postfix/disclaimer

Now lets go and tell Postfix to use the alterMIME filter.  Make sure you backup the existing file incase you need to roll back:

cp /etc/postfix/master.cf /etc/postfix/mastercf.orig

Edit the master.cf file

nano /etc/postfix/master.cf

Right under the first not commented out section lets put the filter in there, so we add a line underneath:

#
# Postfix master process configuration file.  For details on the format
# of the file, see the master(5) manual page (command: “man 5 master”).
#
# Do not forget to execute “postfix reload” after editing this file.
#
# ==========================================================================
# service type  private unpriv  chroot  wakeup  maxproc command + args
#               (yes)   (yes)   (yes)   (never) (100)
# ==========================================================================
smtp      inet  n       –       –       –       –       smtpd
-o content_filter=dfilt:

Scroll all the way to the bottom and add the following to tell Postfix what variables to pass to alterMIME.  Add the following two lines on the bottom.

dfilt     unix    –       n       n       –       –       pipe
flags=Rq user=filter argv=/etc/postfix/disclaimer -f ${sender} — ${recipient}

Save and exit.

No you can go and populate the email lists by typing a seperate email address on it’s own.

nano /etc/postfix/disclaimer_addresses_companya

Add in something like this appropiate to your installation.

companya.com

Feel free to edit your disclaimer:

nano /etc/postfix/disclaimer_companya.txt

You could have something like this in your standard email disclaimer which will be interpreted as HTML.

<br />
The information contained in this document is strictly private & confidential and may also be privileged. If you are NOT the person to whom this email is addressed, please notify the sender immediately by reply. You should not copy the document or any of the attachments which may be contained herein, or use it for any purpose or disclose its contents to any third party.<br />
<br />
Employees of Company A are expressly required not to make any defamatory statements and not to infringe or authorise any infringements of copyright, or any other legal right, by email communication. Any such communication is contrary to the company’s policy and outside the scope of the employment of the individual concerned. The company will not accept any liability in respect of such communication, and the employee will be personally liable for any damages or other liability which may arise from it.<br />
<br />
Company A does not accept responsibility for changes made to this message after it was originally sent.<br />
<br />
Company A is the trading name of Company Alpha Limited, registered office: Company A House, Big Road, Big City, registered in England & Wales under company no 1234567.<br />

Save and close the document and you’re done.

Now reload Postfix:

postfix reload

Give it a test and you should now be attaching the statutory legal requirements onto your email. You may want to edit your email lists and your individual disclaimers too for each company.