Thu 15 August 2019

Postfix Send-Only Mail Service

Posted by Kyle Pericak in systems administration

This post is linked to from the Automated Emails Project

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.

This guide covers how to configure Postfix as a local send-only mail service on a dedicated Ubuntu VM.

Table of contents:

Create a send-only mail server with Postfix
Troubleshooting
- Check for & remove mail stuck in the queue
  - Outlook is blocking your emails
Other References

Create a send-only mail server with Postfix

Find your external IP address

Many VMs use NAT to accesss the internet. Their ifconfig output won't match the actual internet IP that the emails will send from.

To check your external IP address, execute:

dig +short myip.opendns.com @resolver1.opendns.com

Below, the output of this command will be used as <ext_ip>

If you have a static public IP address you can use it as <ext_ip>

Set your hostname to an FQDN in /etc/hosts

This step is only required if you don't already have a local DNS server or internet routed static IP with a valid FQDN.

Lets say that your mail will come from a server named alerts.example.com. Set the hosts file like this:

vi /etc/hosts

127.0.0.1 alerts.example.com
<ext_ip>  alerts.example.com

Install Postfix

This is the software that manages your mail queue and handles SMTP.

apt-get install -y postfix mailutils

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 [email protected], enter alerts.example.com.

Configure Postfix

Here's a configuration that's worked for me. Be sure to change the myhostname field.

vi /etc/postfix/main.cf

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

Once configured, restart Postfix:

systemctl restart postfix

Send a test email from the command-line

Pipe some stdout to the mail command. This will be used as the body of the email.

echo "Test alert please ignore" | mail \
  -a "FROM:[email protected]" \
  -s "Test alert" [email protected]

Troubleshooting

Often emails don't go through. There are lots of reasons that can happen.

Check for & remove mail stuck in the queue

Show the mail queue. It might tell you the error if the email is there.

mailq

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.

To remove items from the mail queue by ID, run

# If you don't pass <ID> it will delete the all.
postsuper -d <ID>

Outlook is blocking your emails

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. - Disable the spam filter - Use a connector (not tested)

Kyle Pericak