Amazon Web Services - AWS EC2 - Ubuntu 14.04

1. Google spam folder & open relay host

Never put relayhost = * in /etc/postfix/main.cf  because your mails will go to Google's SPAM folder.

# tail -f /var/log/syslog

Mar 24 13:53:16 vps postfix/smtp[4852]: 060EB1A0028A: host gmail-smtp-in.l.google.com[2a00:1450:4013:c01::1a] said: 421-4.7.0 [2a02:c200:1:10:2:3:2990:1      15] Our system has detected an unusual 421-4.7.0 rate of unsolicited mail originating from your IP address. To protect 421-4.7.0 our users from spam, mail sent from your IP address has been 421-4.7.0 temporarily rate limited. Please visit 421-4.7.0 http://www.google.com/mail/help/bulk_mail.html to review our Bulk 421 4.7.0 Email Senders Guidelines. eb1si17158529wib.34 - gsmtp (in reply to end of DATA command)