信件退回出現如下訊息

------------------------------------------------

This is the mail system at host gateway31.websitewelcome.com.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

                   The mail system

<XXXXX@OOOOOOO.com>: host ASPMX.L.GOOGLE.com[108.177.8.27] said:
550-5.7.1
    This message does not have authentication information or fails to pass
    550-5.7.1 authentication checks. To best protect our users from spam,
the
    550-5.7.1 message has been blocked. Please visit 550-5.7.1
    https://support.google.com/mail/answer/81126#authentication for more 550
    5.7.1 information. i5si2004963otk.134 - gsmtp (in reply to end of DATA
    command)

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此為google已經改為必須要經過認證的信件才能收信，

必須設定spf認證

請至DNS代管設定新增txt record，在VALUE加入v=spf1 include:websitewelcome.com ~all

因DNS設定須一段時間才會更新,一般主機會說要24小時才會更新，不過可能2、3個小時後就可以測試。

提高MAIL的安全性可以設定伺服器使用SSL認證：
將伺服器的SMTP改為SSL

請將SMTP改為PORT:465，使用SSL加密連線類型
outlook請至檔案->帳戶設定->帳戶設定->變更->其他設定->進階->更改SMTP為465，SSL加密。POP3可以設定SSL PORT965，設為SSL會比較安全。