If you want to execute some special commands, log or make an API call when you’re receiving an email onto your server, you can easily set up this by using procmail. Procmail is designed to filter and sort emails but can make any call you want.

Here, as an example, we will set up a simple call to a python script that will read the content of the mail (headers and body) and put the information into a log file.

1. Install procmail

Depending on the OS you’re using, you should find a package pre-compiled in the common repositories.

For example, on a Debian-based:

apt-get install procmail

or on a CentOS-based:

yum install procmail

2. Build the python script you want to call to analyze the message

You now have to prepare your script you will execute when receiving an email, that will read and parse the content to log interesting information in a file.

Let’s create a script called procmail_script.py

import os
import time
import email
import sys
import logging.handlers
import base64

# Constants
LOG_FILE_PATH = os.path.expanduser('/opt/mailAnalysis.log')

# Set up a logger
my_logger = logging.getLogger('MyLogger')
my_logger.setLevel(logging.INFO)
handler = logging.handlers.RotatingFileHandler(LOG_FILE_PATH,  maxBytes=500000, backupCount=4,)
formatter = logging.Formatter("%(asctime)s - %(levelname)s - %(message)s",  "%Y-%m-%d %H:%M:%S")
handler.setFormatter(formatter)
my_logger.addHandler(handler)

# Main function
def main():
    try:
        # Get message
        full_msg = sys.stdin.read()
        msg = email.message_from_string(full_msg)

        # Prepare dict containing data
        data = {}

        # Fill dict 
        data['From'],e    = email.Header.decode_header(msg['From'])[0] if msg.has_key('From') else ''
        data['To'],e      = email.Header.decode_header(msg['To'])[0] if msg.has_key('To') else ''
        data['Subject'],e = email.Header.decode_header(msg['Subject'])[0] if msg.has_key('Subject') else ''
        data['Body']      = msg.get_payload()

        # Add information to log
        my_logger.info('From:    ' + data['From'])
        my_logger.info('To:      ' + data['To'])
        my_logger.info('Subject: ' + data['Subject'])
        my_logger.info('Body:    ' + data['Body'])
    except Exception,e:
        my_logger.error('----- ERROR ENCOUNTERED')
        my_logger.error(str(e))
        my_logger.error('----- END OF ERROR')

# Main program
if __name__ == "__main__":
    start_time = time.time()
    my_logger.info('----- START: ' + str(time.time()))
    result = main()
    my_logger.info('----- END: ' + str(time.time()))
    my_logger.info('Elapsed Seconds: ' + str(time.time() - start_time))
    handler.close()
    sys.exit(result)

3. Configure your user to pass the mail to your script

So that you can pass rules you want to execute when receiving an email, you need to create a file (hidden) called .procmailrc that will take place in the home directory of the user you want to use.

For example, for executing rules when receiving email to [email protected], you will have to put that file into the home dir like /home/mailuser/.procmailrc

LOGFILE=/var/log/procmail.log
VERBOSE=YES

:0:
* ^Subject:.*procmail.*
* ^[email protected](com|net)
{
  :0c
  procmail-dir/

  :0 fw
  | /usr/bin/python /home/mailuser/procmail_script.py

  :0 e
  procmail-failed-dir/
}

This will perform multiple steps:

1. Check that the mail recipient is [email protected] or [email protected]
2. Put a copy of the email into the procmail-dir directory
3. Pass the message to our python script procmail_script.py
4. Discard the message if the script succeeds (remove from queue) or copy it to procmail-failed-dir if failed (so you can process it later)

4. Prepare an email and perform a testing locally

First, create a sample mail that you will use for testing in a file called procmail_test.txt:

From: [email protected]
To: [email protected]
Subject: This is a procmail testing

Hey there,
I hope this message will be parsed and logged properly as expected.
This is my first test for procmail deployment!

Then, you can test it by executing procmail manually:

procmail VERBOSE=on /home/mailuser/.procmailrc < /home/mailuser/procmail_test.txt

Now, open the file /opt/mailAnalysis.log and you should have something like:

2018-03-31 08:08:45 - INFO - ----- START: 1522570125.06
2018-03-31 08:08:45 - INFO - From:    [email protected]
2018-03-31 08:08:45 - INFO - To:      [email protected]
2018-03-31 08:08:45 - INFO - Subject: This is a procmail testing
2018-03-31 08:08:45 - INFO - Body:    Hey there,
I hope this message will be parsed and logged properly as expected.
This is my first test for procmail deployment!
2018-03-31 08:08:45 - INFO - ----- END: 1522570125.12
2018-03-31 08:08:45 - INFO - Elapsed Seconds: 0.0624470710754