Setting Up Postfix Locally to Send Email via Google SMTP

Want to send and receive email purely through OSS? Here's how to do it.

Sudip Bhandari

CORE ·

May. 20, 19 · Tutorial

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Postfix

Postfix is a free and open-source mail transfer agent (MTA) that routes and delivers email. This is required in order to be able to send emails from a local machine (without using any vendor’s client).

Installation

Postfix has to be configured. The suggested way to do that is via: sudo dpkg-reconfigure postfix. You can also directly edit your /etc/postfix/main.cf file and make changes to the configuration. For most purposes, dpkg-reconfigure should suffice.

Now that we have MTA ready, we need a mail user agent program in order to actually send and receive email. In Unix, the most common tool is mailx.

Installation of mailx

Now given that Postfix is correctly configured, we can start experimenting with mailx. I learned quite a few things doing this and thus I want to enumerate all of my attempts.

mailx -s "test subject" sudip.post@gmail.com
<body>
CtrlD

Command executed successfully. However, I didn’t get the message delivered.

Postfix Logs

Postfix mail logs are generally stored under /var/log/mail.log:

 postfix/smtp[21002]: 0CCFD2083E1: to=<sudip.post@gmail.com>, relay=gmail-smtp-in.l.google.com[172.217.194.26]:25, delay=3.4, delays=0.15/0.11/2.3/0.82, dsn=5.7.1, status=bounced (host gmail-smtp-in.l.google.com[172.217.194.26] said: 550-5.7.1 [122.167.64.74] The IP you\'re using to send mail is not authorized to 550-5.7.1 send email directly to our servers. Please use the SMTP relay at your 550-5.7.1 service provider instead. Learn more at 550 5.7.1  https://support.google.com/mail/?p=NotAuthorizedError s8si140686pgp.140 - gsmtp (in reply to end of DATA command))

I figured out that Google’s SMTP server is not allowing connections via port 25. So, I reconfigured the SMTP port for postfix to be 587. I also added config related to google SMTP authorization.

Postfix Config Changes (StackExchange Source)

# This tells Postfix to hand off all messages to Gmail, and never do direct delivery.
relayhost = [smtp.gmail.com]:587

# This enables TLS (SMTPS) certificate verification, because Gmail has a valid one.
smtp_tls_security_level = verify
smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt
smtp_tls_session_cache_database = btree:/var/run/smtp_tls_session_cache

# This tells Postfix to provide the username/password when Gmail asks for one.
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous

Store Password Inside /etc/postfix/sasl_passwd

[smtp.gmail.com]:587    user@gmail.com:mypassword # inside /etc/postfix/sasl_passwd
postmap /etc/postfix/sasl_passwd #compile password to db (we can safely remove previous cleartext file now)
sudo service postfix restart #restarting as it's run as a service

I tried executing mailx again and still didn’t see that the emails were getting delivered. However, I could see that the mail queue was holding it back. I could see the following error in mail.log.

1917F2083CF: to=<sudip.post@gmail.com>, relay=smtp.gmail.com[74.125.24.109]:587, delay=322, delays=318/0.17/3.3/0, dsn=4.7.9, status=deferred (SASL authentication failed; server smtp.gmail.com[74.125.24.109] said: 534-5.7.9 Application-specific password required. Learn more at?534 5.7.9  https://support.google.com/mail/?p=InvalidSecondFactor o5sm22737898pgc.16 - gsmtp)

That’s when I realized that the password I supplied earlier had to be the ‘app password’ and not the actual Gmail password. A quick Google search got me landed to this official support page. After I generated an app password, I changed the password in /etc/postfix/sasl_passwd, postmapping (to the database) and restarting the Postfix MTA I executed mailx. Voila, I was able to send a message from my Linux terminal using only open source tools:

Sample Postfix Log for Successful Deliveries

postfix/smtp[22127]: 6EC6720842A: to=<sudip.post@gmail.com>, relay=smtp.gmail.com[74.125.24.109]:587, delay=5, delays=0.13/0/3.4/1.4, dsn=2.0.0, status=sent (250 2.0.0 OK  1552745970 p20sm10104430pfi.45 - gsmtp)

Usage for Sys Admins

You can trigger emails based on certain criteria, like when memory usage is high. Here’s one such example I took from Tecmint:

#!/bin/bash 
#######################################################################################
#Script Name    :alertmemory.sh
#Description    :send alert mail when server memory is running low
#Args           :       
#Author         :Aaron Kili Kisinga
#Email          :aaronkilik@gmail.com
#License       : GNU GPL-3  
#######################################################################################
## declare mail variables
##email subject 
subject="Server Memory Status Alert"
##sending mail as
from="server.monitor@example.com"
## sending mail to
to="admin1@example.com"
## send carbon copy to
also_to="admin2@example.com"

## get total free memory size in megabytes(MB) 
free=$(free -mt | grep Total | awk '{print $4}')

## check if free memory is less or equals to  100MB
if [[ "$free" -le 100  ]]; then
        ## get top processes consuming system memory and save to temporary file 
        ps -eo pid,ppid,cmd,%mem,%cpu --sort=-%mem | head >/tmp/top_proccesses_consuming_memory.txt

        file=/tmp/top_proccesses_consuming_memory.txt
        ## send email if system memory is running low
        echo -e "Warning, server memory is running low!\n\nFree memory: $free MB" | mailx -a "$file" -s "$subject" -r "$from" -c "$to" "$also_to"
fi

exit 0

We can have this script run as a cron job, say every 5 minutes or so, and get the job done.

Having a local config and set up of Postfix, mailx and other Unix utilities definitely comes in handy in other such scenarios, too.

Google (verb) Open source

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