Norwegian Church Aid received the yellowed letter in May this year with the words "returned to sender", "address unknown" stamped upon it. It had been sent out in 1975.
"Where it has been and what it has been doing all these years, one can only wonder," Håkon Haugsbø, the charity's press officer told Norway's VG newspaper.
The man to whom the letter was addressed had moved in 1975, before the letter was delivered.
He died in 2005, but VG has contacted his daughter and arranged for the charity to send on the yellowed envelope so that the mail will finally delivered.
"It's funny to think that my father's letter has been gone for almost 40 years," she told the newspaper. "Had he been alive now, he would have been wracked with laughter at this."
Hilde Ebeltoft Skaugrud, a spokeswoman from the Norwegian postal service apologized on behalf of the organization to both NCA and the man's daughter.
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"Most likely it has been misplaced somewhere. It may have fallen out of a container inside one of our terminals, or it may have been left trapped somewhere," she said. "At the same time this story shows that we never give up! Even after 40 years, we got the letter back to the sender."