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SMTP error from remote mail server after end of data: 550-5.7.1

2017-09-08 16:33:08 1dqFeu-0003gC-DH <= sales@usmilekids.com H=(vps.usmilekids.com) [::1] P=esmtpa A=login:sales@usmilekids.com S=561 id=56ef5a71f3fba9a1f2ff2100f106f2c6@usmilekids.com T="testsss" from for burin@netway.co.th
2017-09-08 16:33:10 1dqFeu-0003gC-DH ** burin@netway.co.th F= R=lookuphost T=remote_smtp H=aspmx.l.google.com [2404:6800:4003:c00::1a] X=TLSv1.2:ECDHE-RSA-AES128-GCM-SHA256:128 CV=yes: SMTP error from remote mail server after end of data: 550-5.7.1 This message does not have authentication information or fails to pass\n550-5.7.1 authentication checks. To best protect our users from spam, the\n550-5.7.1 message has been blocked. Please visit\n550-5.7.1 https://support.google.com/mail/answer/81126#authentication for more\n550 5.7.1 information. s80si1314576pfj.66 - gsmtp
2017-09-08 16:33:10 1dqFew-0003gR-NA <= <> R=1dqFeu-0003gC-DH U=mail P=local S=2611 T="Mail delivery failed: returning message to sender" from <> for sales@usmilekids.com
2017-09-08 16:33:10 1dqFeu-0003gC-DH Completed

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ปิด IPV6

1.# nano -w /etc/sysctl.conf

เพิ่มค่า ท้ายสุดของบรรทัด แล้วทำการ save

net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6=1

2.# sysctl -p