How to Set Up Email Signatures with Postfix on Ubuntu Server 16.04

Hello fellow server enthusiasts! Today, I'd like to share a solution to a common request I often come across: setting up an email server to automatically add signatures to user emails. Specifically, we'll look at the Postfix SMTP server running on Ubuntu Server 16.04, and how to integrate signatures using altermime.

Step-by-Step Guide to Adding Signatures with Postfix and Altermime

1. Pre-requisites

• Configured and operational SMTP Postfix server.
• Ubuntu Server 16.04 OS.

2. Switch to Superuser Mode

sudo su

3. Update and Install Necessary Packages

First, ensure your package list is updated.

apt update

apt install altermime

4. Configuration

• Add a dedicated user for altermime:

useradd -r -c "Postfix Filters" -d /var/spool/filter filter

• Set up a directory with the correct permissions:

mkdir /var/spool/filter
chown filter:filter /var/spool/filter/
chmod 755 /var/spool/filter/

5. Implement Mail Processing Script

nano /etc/postfix/disclaimer

#!/bin/sh
# Localize these.
INSPECT_DIR=/var/spool/filter
SENDMAIL=/usr/sbin/sendmail
DISCLAIMERS=/etc/postfix/disclaimers
# 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.$$ || { echo Cannot save mail to file; exit $EX_TEMPFAIL; }
from_address=`grep -m 1 "From:" in.$$ | cut -d "<" -f 2 | cut -d ">" -f 1`
from_address=`echo "$from_address" | sed "s/From: //g"`


if [ -f $DISCLAIMERS/$from_address ] ; then
/usr/bin/altermime --input=in.$$ \
                   --disclaimer=$DISCLAIMERS/${from_address} \
                   --disclaimer-html=$DISCLAIMERS/${from_address} || \
                     { echo Message content rejected; exit $EX_UNAVAILABLE; }
fi
$SENDMAIL -oi "$@" <in.$$

exit $?

This script will manage the email processing and integration of signatures based on sender addresses.

6. Permissions and Storage

After finalizing the script, set the correct permissions:

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

Next, designate a storage location for your signature files:

mkdir /etc/postfix/disclaimers

7. Create Signatures

For every email address that requires a signature, create a signature file:

echo "signature for mailbox [email protected]" >> /etc/postfix/disclaimers/[email protected]
echo "signature for mailbox [email protected]" >> /etc/postfix/disclaimers/[email protected]
chgrp -R filter /etc/postfix/disclaimers
chmod -R 750 /etc/postfix/disclaimers

8. Update Postfix Configuration

Lastly, adapt your Postfix config to recognize and use the new signature setup.

nano /etc/postfix/master.cf

You should add -o content_filter=dfilt: after the line with smtp, like this:

smtp      inet  n       -       y       -       -       smtpd
    -o content_filter=dfilt:

Spaces int the beginning of the line are important.

Also add the following to the end of the file:

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

Remember to restart Postfix after making changes:

/etc/init.d/postfix restart

Conclusion

By following these steps, any messages sent from addresses listed in disclaimers will now include the desired signature. This technique greatly enhances email professionalism and brand consistency.

Note:

When sending plain text emails, ensure you're aware of potential encoding issues with signatures. Default encoding is UTF-8, but adjustments might be necessary based on your email client.

iconv -f UTF-8 -t cp1251 test\@fl-mon.com > tmp
mv tmp test\@fl-mon.com