I get the following message when I tried to send a mail.

host gmail-smtp-in.l.google.com[74.125.129.27] 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.