用outlook寄信給gmail的信件會退信，出現如下訊息

The original message was received at Wed, 6 Dec 2023 11:20:02 +0800 from X-X-X-X.hinet-ip.hinet.net [X.X.X.X]

----- The following addresses had permanent fatal errors ----- <OOOOOO@gmail.com>
(reason: 550-5.7.26 This mail has been blocked because the sender is unauthenticated.)

----- Transcript of session follows ----- ... while talking to gmail-smtp-in.l.google.com.:
>>> DATA
<<< 550-5.7.26 This mail has been blocked because the sender is unauthenticated.
<<< 550-5.7.26 Gmail requires all senders to authenticate with either SPF or DKIM.
<<< 550-5.7.26
<<< 550-5.7.26 Authentication results:
<<< 550-5.7.26 DKIM = did not pass
<<< 550-5.7.26 SPF [YYYY.com] with ip: [Z.Z.Z.Z] = did not pass <<< 550-5.7.26 <<< 550-5.7.26 To mitigate this issue, please visit Gmail's authentication guide <<< 550-5.7.26 for instructions on setting up authentication:
<<< 550 5.7.26 https://support.google.com/mail/answer/81126#authentication a7-20020a655c87000000b005bd66bfc253si5833547pgt.148 - gsmtp
554 5.0.0 Service unavailable

......

但使用webmail寄信則不會出現問題，

SPF和DKIM都已經設定正確了還是會出現錯誤

20年前E-MAIL，因SPAM外寄郵件主機有些會設定成ISP的郵件主機，如HINET會設msa.hinet.net就可順利寄出。

如今gmail會檢查SPF及DKIM，DKIM會檢測寄信的IP，若還設定成ISP的外寄郵件主機，發信的IP就成了ISP的IP，而不是自己網域的郵件主機IP，就會造成檢查無法通過的問題。

所以若SPF及DKIM都設了，請檢查一下OUTLOOK的外寄郵件主機是否設定成自己EMAIL的網域。