The first moogle you meet, Kupo will tell you they are having trouble delivering mail and ask you to give a message to Monty in Evil Forest. Later in the game, he'll give you a message for Mosh.
Final Fantasy IX
Mognet Side Quest
Introduction
Something has gone wrong at SystemMoogle mail side quest chain. Helps track letters and late-game Superslick access. Central! You’ll need to help deliver de letter (de sooner de better!) for all the poor mognetless moogles. To help you out, here is a list of all the moogles you’ll need to find, and who their letters are from/to. Happy delivery! Be sure to check out the rest of the story under “Optional: Mognet” on Disc 4. You’ll also need a chocobo who can open cracks, plus some Dead Peppers to feed him. See the Chocobo Bubbles and Cracks section for more information on this topic.
Disc 1
Receive a message from Ruby to Zidane.
Give mail from Kupo to Monty, then read mail from Stiltzkin to Monty.
Thaw the ice and get mail from Mois to Gumo.
Give mail from Mois to Gumo.
Get a letter from Kumop to Mogki.
Give Mogki the letter from Kumop, then get a letter to Atla.
Get a letter from Ruby to Zidane.
Read a letter from Stiltzkin.
Read a letter from Moodon.
Give Atla a letter from Mogki, then get a letter for Monev.
Disc 2
Get a letter for Nazna.
Give Nazna a letter from Grimo, then get a letter for Mochos.
Read a letter from Stiltzkin.
Give Mochos a letter from Nazna.
Give Monev a letter from Atla in exchange for a Kupo Nut.
Read a letter from Ruby to Zidane.
Get a letter from Mopli to Serino.
Give Serino a letter from Mopli, then get a letter to deliver to Moodon.
Get a letter for Monty.
Give Monty a letter from Mosh.
Give Moodon a letter from Serino, read a letter from Ruby, and get a letter for Moonte.
Give Moonte a letter from Moodon.
Read a letter from Kuppo.
Use the hammer to let Kuppo out, then get a letter for Kupo from Kuppo.
Get a letter from Mogmatt to Suzuna.
Get a letter from Mogryo to Mocchi.
Give Suzuna a letter from Mogmatt.
Give Mocchi a letter from Mogryo.
Read a letter from Stiltzkin.
Disc 3
Give Kupo a letter from Kuppo, then read a letter from Mogrika to Kupo. After you become Zidane, head back and read a letter from Stiltzkin.
Read a letter from Kupo to Mogrich.
Get a letter from Mogki to Moodon.
Give Moodon a letter from Mogki.
Get a letter for Mooel.
Give Mooel a letter from Mimoza and get a Kupo Nut.
Get a letter from Mojito to Mogsam.
Give Mogsam a letter from Mojito.
Read a letter from Artemicion to Mogrika, then get a letter from Mogrika to Moolan.
Give Moolan a letter from Mogrika, then get a letter from Moolan to Mogtaka.
Get a letter from Mogki to Kumool.
Give Kumool a letter from Mogki.
Get a letter from Moorock to Mozme.
Give Mozme a letter from Moorock.
Disc 4
Don't Miss: This is the late Mognet cleanup chain. You'll need access to Mognet Central, which means Choco needs the ability to open cracks and you need Dead Peppers. See Bubbles and Cracks for more info.
Get a letter for Atla.
Give Atla a letter from Kupo, then get a Kupo Nut and a letter for Mogryo.
Give Mogryo a letter from Atla, then get a Kupo Nut and a letter for Kumool.
Give Kumool a letter from Mogryo, then get a letter for Mois.
Bring Quina, give Mois the letter from Kumool, then get a letter for Noggy.
Give Noggy a letter from Mois, then get a letter for Kupo.
Give Kupo a letter from Noggy.
She's not a mog, but talk to her anyway.