OSSEC postfix email using localhost doesn't work

OSSEC had issues sending me emails with the following error message in the /var/ossec/logs/ossec.log

ERROR: Error Sending email to localhost (smtp server)

OSSEC was configured to use POSTFIX as my SMTP host as configured in the /var/ossec/etc/ossec-server.conf

  <global>  
   <email_notification>yes</email_notification>  
   <email_to>philip.wu@anu.edu.au</email_to>  
   <smtp_server>localhost</smtp_server>  
   <email_from>patient-lookup@130.56.244.180</email_from>  
  </global>  

Once I changed localhost to 127.0.0.1, postfix emails worked:

  <global>  
   <email_notification>yes</email_notification>  
   <email_to>philip.wu@anu.edu.au</email_to>  
   <smtp_server>127.0.0.1</smtp_server>  
   <email_from>patient-lookup@130.56.244.180</email_from>  
  </global>  

Reference:

https://github.com/ossec/ossec-hids/issues/1122