Here is the full list of all 23 Mail Time achievements. It takes around 1-2 hours to unlock all of the achievements on Windows.
you did it!! kinda... not really.... BUT you did do other things!
glide for 15 minutes!
deliver meals instead of mail!
deliver a fan's letter to their idol!
complete 50 deliveries!
find 40 trinkets!
sample all the rare mushrooms!
deliver an anonymous love letter <3
meet everyone in the neighbourhood!
do your part & clean up the forest!
glide for 5 minutes!
prevent a crime from taking place!!
complete 25 deliveries!
find 20 trinkets!
deliver a letter in under 30 seconds!
take over 15 minutes to deliver a letter!
find all rare coins!
complete 10 deliveries!
glide for 60 seconds!
find 5 trinkets!
file an official letter of complaint >:(
get to know your neighbourhood
Deliver your very first letter!