gmail拒收邮件错误:550-5.7.1

<receive@gmail.com>: host gmail-smtp-in.l.google.com[2607:f8b0:400e:c02::1b]<br />
    said: 550-5.7.1 [2400:8900::f03c:91ff:fe70:8dd1       7] Our system has<br />
    detected that 550-5.7.1 this message is likely unsolicited mail. To reduce<br />
    the amount of spam 550-5.7.1 sent to Gmail, this message has been blocked.<br />
    Please visit 550-5.7.1<br />
    http://support.google.com/mail/bin/answer.py?hl=en&answer=188131 for 550<br />
    5.7.1 more information. pu9si40114837pbb.303 – gsmtp (in reply to end of<br />
    DATA command)

<receive@gmail.com>: host gmail–smtp–in.l.google.com[2607:f8b0:400e:c02::1b]

said: 550–5.7.1 [2400:8900::f03c:91ff:fe70:8dd1 7] Our system has

detected that 550–5.7.1 this message is likely unsolicited mail. To reduce

the amount of spam 550–5.7.1 sent to Gmail, 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. pu9si40114837pbb.303 – gsmtp (in reply to end of

DATA command)

起初检查了邮件系统，毕竟很久没更新了。甚至在本机虚拟机上重新假设了一个新的邮件系统，发现效果依然如此。百度了大半天，基本都是没有用的垃圾信息。
最后将“Our system has detected that 550-5.7.1 this message is likely unsolicited mail. ”放到谷歌上搜索，发现有很多人有这种情况，不过更多的人是IP进入了谷歌的黑名单，而我们的知识因为邮件服务器域名未验证的原因，然后继续谷歌继续百度，原来DNS设置里有个TXT记录（TXT record）需要设置。

v=spf1 a mx ip4:8.8.8.8 -all
大概意思是spf记录1版本（到现在似乎就只有1个版本），DNS中的a记录，mx记录，特别是IP4等于指定的例如这里设置的8.8.8.8（你的服务器地址）都是经过域名所有者认证的邮件。