During the reign of Emperor Wu of Han, Su Wu was sent as an envoy to the Xiongnu. Later, the Xiongnu's Chanyu forcibly detained him at Lake Baikal under a pretext. Su Wu remained unyielding, so he was secretly sent to tend sheep. The Chanyu told him, "When the ram gives birth to a lamb, you may return to the Han court." The implication was that Su Wu would never go back.
During the reign of Emperor Zhao of Han, the Xiongnu and Han Dynasty were allied through marriage. The Han court demanded the release of Su Wu, but the Xiongnu chanyu lied to the Han envoy, claiming Su Wu had died. A Han envoy named Chang Hui, who had accompanied Su Wu on his earlier mission to the Xiongnu, secretly met with the Han envoy and revealed that Su Wu was herding sheep at Lake Baikal. Chang Hui then devised a plan: "Tell the chanyu that the Han emperor was hunting at Shanglin Park and shot a wild goose flying south from the north. Tied to its foot was a letter in Su Wu's own hand, saying he is herding sheep at Lake Baikal." This clever ruse exposed the Xiongnu's deception. Source: *Records of the Grand Historian*, "Biography of Su Wu"
The Han envoy, delighted by Chang Hui's clever strategy, presented it to the Chanyu, who had no choice but to admit defeat and release Su Wu back to the Han court.
Later, the idiom "Wild Goose Delivers a Letter" came to describe sending correspondence.
Source: *Book of Han*, "Biography of Su Wu"
Meaning of the Idiom: Later, the Chinese idiom "雁足传书" came to describe how sending correspondence.