A courier ninja's job is similar to that of postmen in that they deliver mail. According to Courier Ninja 596-03, they are professionals who risk their lives to deliver goods to the customers. The only differences between postmen and courier ninja are that postmen don't risk their lives while courier ninja do. Letters and packages are also much safer with courier ninja as they are known for their efficiency and outstanding records.
On Naruto's way back to Konoha from a recent mission, Naruto stops at a village with a hot springs resort. Along the way, he meet a unique ninja called a Courier Ninjawho's mission duties were solely to deliver mail through the most difficult and dangerous of terrains. At the village, his shock and fury Naruto discovers Jiraiya fooling around with some girls. To calm Naruto down, Jiraiya reveals the real reason he hasn't started Naruto's training yet was because he was on a mission to gather information on the Akatsuki and learned that an agent of theirs was reported to be arriving in the village soon. Naruto asked Jiraiya to train him since they had the time, but Jiraiya explained he also had to finish the manuscript for his latest novel. After constant procrastination on Jiraiya's part, he eventually fell asleep with his work unfinished. Naruto instead decided to ghostwrite the manuscript for Jiraiya and had a courier ninja deliver it, after bumping into a couple men and getting their mail mixed together.
When Naruto told Jiraiya what he did, Jiraiya was furious as he feared this would ruin his career as an author. After getting the mail back, Jiraiya was horrified to discover that the package contained a peace treaty between the Land of Fang and Claw who were on the brink of war. Realising that Naruto's manuscript was being sent to the Fang Daimyō and fearing that the Fang Daimyō would view as an insult for war, Jiraiya and Naruto rushed to get the manuscript back. After a heated struggle against the courier ninja, the Fang Daimyō unfortunately got the manuscript. But to Naruto and Jiraiya's shock, the Fang Daimyō was a huge fan of the Icha Ichaseries and thought it was a peace offering from the Land of Claw, effectively causing the war to be cancelled. Later, Jiraiya had to leave as his Akatsuki target had already left the hot springs resort, forcing Naruto to return to Konoha to wait for Jiraiya's return.