Thread: Permanent Error
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Old 12 Feb 2022, 02:00 AM   #16
Intergalactic Postmaster
Join Date: May 2004
Location: Irving, Texas
Posts: 8,781
Iím sure that in this case, the Yahoo delivery server kept a copy of the message and also forwarded a copy to a Gmail address. See the Yahoo premium account setup (forwarding was discontinued from free Yahoo accounts on January 15, 2021, but is still available for premium accounts):

So one copy of the will remain in the Yahoo user Inbox, while an attempt was made to forward a copy to the Gmail account.
  • If the Gmail server refused to accept the message (which is normally the case with a permanent error), a Yahoo server would send a delivery failure message to the original message sender (From header address).
  • If the Gmail server accepted the message but could not complete delivery for some reason (perhaps an account over quota), a Gmail server might send the delivery failure message to the From address.
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