Email Sending Error

Sharing the errors you are getting, the version of WordPress you're using, the distro (and version) that you are using, etc would be helpful for someone trying to help.

this is the error message that I got from /Mailbox/new/Emailfilename13454

  demo_user@gmail.com
    SMTP error from remote mail server after end of data:
    host gmail-smtp-in.l.google.com [2607:f8b0:400d:c06::1a]:
    550-5.7.1 [2600:3c03::f03c:91ff:fe67:ec43] Our system has detected that this
    550-5.7.1 message does not meet IPv6 sending guidelines regarding PTR records
    550-5.7.1 and authentication. Please review
    550-5.7.1  https://support.google.com/mail/?p=ipv6_authentication_error for more
    550 5.7.1 information. f128si5179812qhe.29 - gsmtp

------ This is a copy of the message, including all the headers. ------

Return-path: <tester@linuxbox.creativeocean.net>Received: from tester by linuxbox.creativeocean.net with local (Exim 4.76)
        (envelope-from <tester@linuxbox.creativeocean.net>)
        id 1Zs6aV-0004G6-KF
        for hlaporthein@gmail.com; Fri, 30 Oct 2015 16:37:11 +0630
Subject: Testing
To: <demo_user@gmail.com>X-Mailer: mail (GNU Mailutils 2.2)
Message-Id: <e1zs6av-0004g6-kf@linuxbox.creativeocean.net>From: tester_leader <tester@linuxbox.creativeocean.net>Date: Fri, 30 Oct 2015 16:37:11 +0630

This is a test.</tester@linuxbox.creativeocean.net></e1zs6av-0004g6-kf@linuxbox.creativeocean.net></demo_user@gmail.com></tester@linuxbox.creativeocean.net></tester@linuxbox.creativeocean.net> 

This is how I configure the hosts file

Here is my hosts file configuration from /etc/hosts, 
127.0.0.1       localhost.localdomain localhost
127.0.0.1       linuxbox.creativeocean.net      linuxbox
45.XX.XXX.45    mail.creativeocean.net  mail

# The following lines are desirable for IPv6 capable h
::1     ip6-localhost ip6-loopback

Please help me, Thanks in advanced

There may be more than one issue but so far, this error message:
> 550-5.7.1 [2600:3c03::f03c:91ff:fe67:ec43] Our system has detected that this

550-5.7.1 message does not meet IPv6 sending guidelines regarding PTR records
is basically saying that you don't have reverse DNS configured for your Linode's IPv6 address. Google is complaining about it and wants you to fix it.

Here's some information we have on reverse DNS (PTR) records: https://www.linode.com/docs/networking/ … everse-dns">https://www.linode.com/docs/networking/dns/setting-reverse-dns