Sending email with Mailgun

Google Cloud works with Mailgun to provide an email service that has a programmatic API, log retention, email personalization, analytics, and email validation.

The following instructions show you how to configure Mailgun as an email relay with Postfix.

Before you begin

Sign up for and create a new Mailgun account on the Google Cloud Marketplace.
Get your credentials. The instructions require that you know your Mailgun SMTP username, password, and hostname. Get your username and password from the Mailgun control panel, under the Domains section.

Depending on how the domain is configured in Mailgun, the SMTP hostname is either smtp.mailgun.org or smtp.eu.mailgun.org.
Configure your firewall rules to allow outgoing traffic on TCP port 2525.

Configuring Mailgun as a mail relay with Postfix

Configuring Mailgun as a mail relay allows the Postfix mail transfer agent to forward emails destined for remote delivery.

Connect to your instance using SSH.
```
gcloud compute ssh [INSTANCE_NAME]
```
where [INSTANCE_NAME] is the name of the VM instance where you want to send email from.

Become a superuser and set a safe umask.

user@test-instance:~$ sudo su -

root@test-instance:~# umask 077

Install the Postfix Mail Transport Agent.

Debian

root@test-instance:~# apt update && apt -y install postfix libsasl2-modules

CentOS

root@test-instance:~# yum install postfix cyrus-sasl-plain cyrus-sasl-md5 -y

When prompted, select the Local Only configuration and accept the default choices for domain names.
Modify the Postfix configuration options. Postfix configuration options are set in the main.cf file. Open the file with the text editor of your choice.
```
root@test-instance:~# vi /etc/postfix/main.cf
```

If they exist, comment out the following lines.

# default_transport = error
# relay_transport = error

Add the Mailgun SMTP service by adding the following line to the end of the file.
```
relayhost = [smtp.mailgun.org]:2525
```
Note: You must use port 2525 because port 25 isn't allowed on Compute Engine.
To enforce SSL/TLS support and configure SMTP authentication for these requests, add the following lines to the end of the file. A simple access and security layer (SASL) module handles authentication in the Postfix configuration.
```
smtp_tls_security_level = encrypt
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous
```
Save your changes and close the file.
Generate the SASL password map.
1. Create a new password file that is ready for standard input.
```
root@test-instance:~# cat > /etc/postfix/sasl_passwd << EOF
```
2. At the prompt, enter the service details, replacing YOUR_SMTP_LOGIN and YOUR_SMTP_PASSWORD with your credentials. See the Mailgun help for instructions on how to view or change your per-domain credentials.
```
> [smtp.mailgun.org]:2525 YOUR_SMTP_LOGIN:YOUR_SMTP_PASSWORD
```
3. Close and save the file by typing the delimiter, EOF.
```
> EOF
```

Use the postmap utility to generate a .db file.

root@test-instance:~# postmap /etc/postfix/sasl_passwd

root@test-instance:~# ls -l /etc/postfix/sasl_passwd*

-rw------- 1 root root    68 Jun  1 10:50 /etc/postfix/sasl_passwd
-rw------- 1 root root 12288 Jun  1 10:51 /etc/postfix/sasl_passwd.db

Next, remove the file that contains your credentials because it is no longer needed.
```
root@test-instance:~# rm /etc/postfix/sasl_passwd
```

Set the permissions on your .db file.

root@test-instance:~# chmod 600 /etc/postfix/sasl_passwd.db

root@test-instance:~# ls -la /etc/postfix/sasl_passwd.db

-rw------- 1 root root 12288 Aug 31 18:51 /etc/postfix/sasl_passwd.db

Finally, reload your configuration to load the modified parameters.

Debian

root@test-wheezy:~# /etc/init.d/postfix restart

CentOS

[root@test-centos ~]# postfix reload

Test your configuration. Install the mailx or mailutils package and test your configuration.

Debian

root@test-wheezy:~# apt -y install mailutils

CentOS

[root@test-centos ~]# yum install mailx -y

Send a test message.

root@test-instance:~# echo 'Test passed.' | mail -s 'Test-Email' EMAIL@EXAMPLE.COM

Look in your systems logs for a status line containing status and the successful server response code (250).

Debian

root@test-wheezy:~# tail -n 5 /var/log/syslog

CentOS

[root@test-centos ~]# tail -n 5 /var/log/maillog

For detailed examples and information about other topics including tracking and routing messages, read the Mailgun documentation.

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