Below outlines the issue and resoultion when trying to send email using Postfix.
Issue
When trying to send an email on my linux platform I get the following error message in my mail.log ??
The IP you’re using to send mail is not authorized 550-5.7.1 to send email directly to our servers. Please use the SMTP 550-5.7.1 relay at your service provider instead. Learn more at 550 5.7.1 http://mail.google.com/support/bin/answer.py?answer=10336 u14si20903226gvf.6 (in reply to end of DATA command))
Resolution
This is down google thinking that you could poss be a spammer.
1. Add the following line to your /etc/postfix/main.cf file,
relayhost = <ISP Relay Server Name>
Change the server name to your ISP relays server name.
2. Restart postfix – ‘/etc/init.d/postfix restart’
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