退信主旨 : Undelivered Mail Returned to Sender

而不是退給轉寄者

expanded from

或是 Mail SMTP Log

to=abc.cde@gmail.com, relay=alt1.gmail-smtp-in.l.google.com[142.250.141.27]:25, delay=2.9, delays=0.05/0/2.1/0.68, dsn=4.7.26, status=deferred (host alt1.gmail-smtp-in.l.google.com[142.250.141.27] said: 421-4.7.26 Your email has been rate limited because it is unauthenticated. Gmail 421-4.7.26 requires all senders to authenticate with either SPF or DKIM. 421-4.7.26 421-4.7.26 Authentication results: 421-4.7.26 DKIM = did not pass 421-4.7.26 SPF [test1.ublink.org] with ip: [168.95.4.211] = did not 421-4.7.26 pass 421-4.7.26 421-4.7.26 For instructions on setting up authentication, go to 421 4.7.26 https://support.google.com/mail/answer/81126#authentication 98e67ed59e1d1-2dbb9defafdsi1432153a91.183 - gsmtp (in reply to end of DATA command))

說明
https://support.google.com/mail/answer/81126#authentication 98e67ed59e1d1-2dbb9defafdsi1432153a91.183

因為 GMail 因為 SPF 是檢查原寄件者來源IP的SPF設定
因為經過轉寄
因次判定為偽造網域
寄件者的郵件伺服器不可能幫你加入 include spf 你的 Mail Server IP