Email error 550: 550 not permitted to relay through this server without authentication

Have you ever came across with the following error while trying to send mail to other mail account for eg, test@example.com.

2015-01-08 17:34:33 H=mail-qc0-f193.google.com [x.x.x.x]:34198 X=TLSv1:RC4-SHA:128 F=<test@gmail.com> rejected RCPT <test@example.com>: Please turn on SMTP Authentication in your mail client.  mail-qc0-f193.google.com [x.x.x.x]:34198 is not permitted to relay through this server without authentication.

I got this error while trying send mail from our test mail account to other mail account for eg, test@example.com
I have tried a lot and got one solution 

Solution:

Check the text file /etc/localdomains and check to make sure the domains having problems are present in the list. If not, then add them to the list save the file and then Restart Exim for the changes to take effect.

Please note that always check /etc/localdomains when having email problems.

That’s it