Configure Postfix to Send Mail Using an External SMTP Server(Relay Postfix mails via smtp.gmail.com)

Introduction:

This guide will help you to use a Gmail account as a free SMTP server on your Ubuntu-Linux server 14.04. Once configured, all emails from your server will be sent via Gmail. This method will be useful if you have many sites on your server and want them all to send emails via Gmail’s SMTP server.

There are many reasons why you would want to configure Postfix to send email using an external SMTP provider such as Google Apps (Gmail), Mandrill, SendGrid, Amazon SES, or any other SMTP server. One reason is to avoid getting your mail flagged as spam if your current server’s IP has been added to a spam list.

Prerequisites

Before starting this tutorial, you should have:
  • Debian 7 installed on your Linode
  • Your fully qualified domain name (FQDN)
  • A valid username and password for the SMTP mail provider, such as Google Apps, Mandrill, or SendGrid
  • Make sure the libsasl2-modules package is installed and up to date:
       sudo apt-get install libsasl2-modules


open the /etc/postfix/main.cf file with your favorite text editor:

#vim /etc/postfix/main.cf
Make sure that the myhostname parameter is configured with your server’s FQDN:

myhostname = mail.xm.shv

Configuring SMTP Usernames and Passwords

#vim /etc/postfix/sasl_passwd




Add your destination (SMTP Host), username, and password in the following format:




[mail.isp.example]:587 username:password






example with gmail:  [smtp.gmail.com]:587 xm.alert@gmail.com:password









Create the hash db file for Postfix by running the postmap command:



#postmap /etc/postfix/sasl_passwd



If all went well, you should have a new file named sasl_passwd.db in the /etc/postfix/ directory.






Securing Your Password and Hash Database Files

The /etc/postfix/sasl_passwd and the /etc/postfix/sasl_passwd.db files created in the previous steps contain your SMTP credentials in plain text.

For security reasons, you should change their permissions so that only the root
 user can read or write to the file. Run the following commands to 
change the ownership to root and update the permissions for the two 
files:












#chown root:root /etc/postfix/sasl_passwd /etc/postfix/sasl_passwd.db
#chmod 0600 /etc/postfix/sasl_passwd /etc/postfix/sasl_passwd.db



Configuring the Relay Server

In this section, you will configure the /etc/postfix/main.cf file to use the external SMTP server.


Open the /etc/postfix/main.cf file with your favorite text editor:




#vim /etc/postfix/main.cf






Update the relayhost parameter to show your external SMTP relay host. Important: 


If you specified a non-default TCP port in the sasl_passwd file, 


then you must use the same port when configuring the relayhost parameter.
















# specify SMTP relay host 
relayhost = [mail.isp.example]:587








At the end of the file, add the following parameters to enable authentication:




/etc/postfix/main.cf




# enable SASL authentication 
smtp_sasl_auth_enable = yes
# disallow methods that allow anonymous authentication. 
smtp_sasl_security_options = noanonymous
# where to find sasl_passwd
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
# Enable STARTTLS encryption 
smtp_use_tls = yes
# where to find CA certificates
smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt




  • Save your changes.
    • 

  • Restart Postfix:
    • 
#/etc/init.d/postfix restart
    

    

    

    

Testing Postfix
    
The fastest way to test your configuration is to send an email to any unrelated email address, using the mail command:
    

    

    echo "body of your email" | mail -s "This is a Subject" -a 
    

    "From: you@example.com" recipient@elsewhere.com
    

    

    Alternatively, you can use Postfix’s own sendmail implementation, 
    

    by entering lines similar to those shown below:
    

    

    sendmail recipient@elsewhere.com
From: you@example.com
Subject: Test mail
This is a test email
    

     
    

    
Troubleshooting
    
Error: “SASL authentication failed; server smtp.gmail.com”
    
You need to unlock the captcha by visiting this page https://www.google.com/accounts/DisplayUnlockCaptcha
    
You can run test again after unlocking captcha.
    

    

    

    =========================================
    

    Reference:
    

    https://easyengine.io/tutorials/linux/ubuntu-postfix-gmail-smtp/ 
    

    

    -https://www.linode.com/docs/email/postfix/postfix-smtp-debian7
    

    

    

    

    

    

    



    

    

        
    
      


    

    


    

    

    
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