Configure Postfix to relay all emails via SMTP gateway - Documentation

1. Configure Postfix

Edit main.cf file

sudo vi /etc/postfix/main.cf

Ensure the following values are set as shown below

mail_owner = _postfix

setgid_group = _postdrop

And add the following lines at the end of the file

relayhost=smtp.gmail.com:587

smtp_sasl_auth_enable=yes

smtp_sasl_password_maps=hash:/etc/postfix/sasl_passwd

smtp_sasl_security_options=noanonymous

smtp_sasl_mechanism_filter=plain

smtp_use_tls=yes

smtp_tls_security_level=encrypt

tls_random_source=dev:/dev/urandom



transport_maps = pcre:/etc/postfix/transport_maps

2. Generating the app password

Goto your gmail account. Click on your profile picture, from the dropdown select Manage your Google Account. Select Security tab from the right hand menu. In the Signing in to Google section, enable 2-Step Verification & then from the App Password generate a password

3. Create a credentials file

This file would be a combination of the SMTP endpoint as well as your email and password to authenticate the SMTP calls.

sudo sh -c 'echo "\nsmtp.gmail.com:587 [email protected]:your_password" >> /etc/postfix/sasl_passwd'

Replace both the email to your actual email and the password to your app password generated earlier in step 2.

Add the credential file to Postfix lookup table

sudo postmap /etc/postfix/sasl_passwd

4. Reloading Postfix

Reload the postfix

sudo postfix reload

5. Testing

Now that we have configured Postfix, created credential file, added it to the Postfix lookup, reloaded the Postfix service. We can now test it by sending an email from our system. It can be tested several way one of which would be to send a test email via command-line as below

echo "This is a test email body." | mail -s "Test Subject" [email protected]

6. Debugging

To check the mail queue list

postqueue -p  # OR

mailq

To delete email from the queue

sudo postsuper -d QUEUE_ID # OR to delete all

sudo postsuper -d ALL

To flush the mail queue

postqueue -f # OR

sudo postfix flush

7. What's Next

Sending test emails from local machine is great however it is a good idea to stop any emails from the local machine going out to the actual clients. To block such emails. Follow the following steps

Since we have the following line added already

transport_maps = pcre:/etc/postfix/transport_maps

Let's create that file with

sudo vi /etc/postfix/transport_maps

and add the following lines to stop emails to all emails expect the one allowed

/^(?!USERNAME).*\@.*\.com$/ discard:

/.*/ :

In the above regex, change the USERNAME to your own username. It will match all emails and discard those except the one with the username added with any hostname at the end. for example

[email protected] # will not match

[email protected] # will match and be discarded