Google blocking mails sent from my server
I get the following message when I tried to send a mail.
host gmail-smtp-in.l.google.com[188.8.131.52] said: 550-5.7.1 [128.199.xxx.xxx 12] Our system has detected that this message is 550-5.7.1 likely unsolicited mail. To reduce the amount of spam sent to Gmail, 550-5.7.1 this message has been blocked. Please visit 550-5.7.1 http://support.google.com/mail/bin/answer.py?hl=en&answer=188131 for 550 5.7.1 more information. sc7si12058958pac.156 - gsmtp (in reply to end of DATA command)
I'm not sure how to resolve this. Anyone has experience with this?
Using Ubuntu 14.04, Postfix, PHP to send mails.
Followed the tutorial How To Install and Setup Postfix on Ubuntu 14.04.