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!
deliver an anonymous love letter <3
find 40 trinkets!
sample all the rare mushrooms!
prevent a crime from taking place!!
find all rare coins!
complete 25 deliveries!
deliver a letter in under 30 seconds!
meet everyone in the neighbourhood!
glide for 5 minutes!
complete 10 deliveries!
find 20 trinkets!
do your part & clean up the forest!
take over 15 minutes to deliver a letter!
glide for 60 seconds!
file an official letter of complaint >:(
get to know your neighbourhood
find 5 trinkets!
Deliver your very first letter!