<p>In order to send emails from a VM you need a mail server. Often you can use
a public, corproate, or cloud mail server, but sometimes you might need to
deploy your own locally.</p>
<p>This guide covers how to configure Postfix as a local send-only mail service on
a dedicated Ubuntu VM.</p>
<hr>
<h3 id="table-of-contents">Table of Contents</h3>
<ul>
<li><a href="#create-a-send-only-mail-server-with-postfix">Create a send-only mail server with Postfix</a>
<ul>
<li><a href="#find-your-external-ip-address">Find your external IP address</a></li>
<li><a href="#set-your-hostname-to-an-fqdn-in-etchosts">Set your hostname to an FQDN in /etc/hosts</a></li>
<li><a href="#install-postfix">Install Postfix</a></li>
<li><a href="#configure-postfix">Configure Postfix</a></li>
<li><a href="#send-a-test-email-from-the-command-line">Send a test email from the command-line</a></li>
</ul>
</li>
<li><a href="#troubleshooting">Troubleshooting</a>
<ul>
<li><a href="#check-for--remove-mail-stuck-in-the-queue">Check for &#x26; remove mail stuck in the queue</a>
<ul>
<li><a href="#outlook-is-blocking-your-emails">Outlook is blocking your emails</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="#other-references">Other References</a></li>
</ul>
<h1 id="create-a-send-only-mail-server-with-postfix">Create a send-only mail server with Postfix</h1>
<h2 id="find-your-external-ip-address">Find your external IP address</h2>
<p>Many VMs use NAT to accesss the internet. Their <code>ifconfig</code> output won't match
the actual internet IP that the emails will send from.</p>
<p>To check your external IP address, execute:</p>
<pre><code class="language-bash">dig +short myip.opendns.com @resolver1.opendns.com
</code></pre>
<p>Below, the output of this command will be used as <code>&#x3C;ext_ip></code></p>
<p>If you have a static public IP address you can use it as <code>&#x3C;ext_ip></code></p>
<h2 id="set-your-hostname-to-an-fqdn-in-etchosts">Set your hostname to an FQDN in /etc/hosts</h2>
<p>This step is only required if you don't already have a local DNS server or
internet routed static IP with a valid FQDN.</p>
<p>Lets say that your mail will come from a server named alerts.example.com.
Set the hosts file like this:</p>
<p><code>vi /etc/hosts</code></p>
<pre><code class="language-text">127.0.0.1 alerts.example.com
&#x3C;ext_ip>  alerts.example.com
</code></pre>
<h2 id="install-postfix">Install Postfix</h2>
<p>This is the software that manages your mail queue and handles SMTP.</p>
<pre><code class="language-bash">apt-get install -y postfix mailutils
</code></pre>
<p>If it asks you which type of server to install, select Internet Site.
Give it an FQDN when prompted. If you will be sending emails from
<a href="mailto:noreply@alerts.example.com">noreply@alerts.example.com</a>, enter <code>alerts.example.com</code>.</p>
<h2 id="configure-postfix">Configure Postfix</h2>
<p>Here's a configuration that's worked for me.
Be sure to change the <code>myhostname</code> field.</p>
<p><code>vi /etc/postfix/main.cf</code></p>
<pre><code class="language-ini">myhostname = alerts.example.com

smtpd_banner = $myhostname ESMTP $mail_name (Ubuntu)
biff = no
append_dot_mydomain = no
readme_directory = no
smtpd_tls_cert_file = /etc/ssl/certs/ssl-cert-snakeoil.pem
smtpd_tls_key_file = /etc/ssl/private/ssl-cert-snakeoil.key
smtpd_use_tls = yes
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
mydestination = $myhostname, 63ec22ec9fc1, localhost.localdomain, , localhost
relayhost =
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
mailbox_size_limit = 0
recipient_delimiter = +
inet_interfaces = loopback-only
inet_protocols = all
</code></pre>
<p>Once configured, restart Postfix:</p>
<pre><code class="language-bash">systemctl restart postfix
</code></pre>
<h2 id="send-a-test-email-from-the-command-line">Send a test email from the command-line</h2>
<p>Pipe some stdout to the <a href="https://linux.die.net/man/1/mail">mail</a> command.
This will be used as the body of the email.</p>
<pre><code class="language-bash">echo "Test alert please ignore" | mail \
  -a "FROM:noreply@alerts.example.com" \
  -s "Test alert" kyle@example.com
</code></pre>
<hr>
<h1 id="troubleshooting">Troubleshooting</h1>
<p>Often emails don't go through. There are lots of reasons that can happen.</p>
<h2 id="check-for--remove-mail-stuck-in-the-queue">Check for &#x26; remove mail stuck in the queue</h2>
<p>Show the mail queue. It might tell you the error if the email is there.</p>
<pre><code class="language-bash">mailq
</code></pre>
<p>If you've got a scheduled job that is adding emails, it can fill the queue up
and make you look like a spammer. Some mail servers will rate limit a sender,
especially if they haven't set up mechanisms by which they can be trusted.</p>
<p>To remove items from the mail queue by ID, run</p>
<pre><code class="language-bash"># If you don't pass &#x3C;ID> it will delete the all.
postsuper -d &#x3C;ID>
</code></pre>
<h3 id="outlook-is-blocking-your-emails">Outlook is blocking your emails</h3>
<p>If you get a 500 error from outlook.*, it could need a connector or a rule
exception for the filter. I haven't tested the connector.</p>
<ul>
<li><a href="https://docs.sophos.com/central/Customer/help/en-us/central/Customer/tasks/bypassingexchange.html">Disable the spam filter</a></li>
<li><a href="https://docs.microsoft.com/en-us/exchange/mail-flow-best-practices/use-connectors-to-configure-mail-flow/set-up-connectors-to-route-mail">Use a connector (not tested)</a></li>
</ul>
<h1 id="other-references">Other References</h1>
<ul>
<li><a href="https://www.digitalocean.com/community/tutorials/how-to-install-and-configure-postfix-as-a-send-only-smtp-server-on-ubuntu-16-04">Postfix install guide from Digital Ocean</a></li>
<li><a href="https://hostadvice.com/how-to/how-to-setup-postfix-as-send-only-mail-server-on-an-ubuntu-18-04-dedicated-server-or-vps/">Postfix setup guide from hostadvice.com</a></li>
<li><a href="http://www.postfix.org/BASIC_CONFIGURATION_README.html">Basic Postfix config referece</a></li>
<li><a href="http://www.postfix.org/postconf.5.html">Detailed Postfix config docs</a></li>
<li><a href="https://www.cyberciti.biz/faq/how-to-find-my-public-ip-address-from-command-line-on-a-linux/">Whats My IP from Curl</a></li>
<li><a href="https://tecadmin.net/ways-to-send-email-from-linux-command-line/">5 ways to send email from cli</a></li>
<li><a href="https://www.digitalocean.com/community/tutorials/how-to-install-and-configure-dkim-with-postfix-on-debian-wheezy">DigitalOcean DKIM guide</a></li>
</ul>


Kyle Pericak

Postfix Send-Only Mail Service