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!
meet everyone in the neighbourhood!
find 40 trinkets!
complete 50 deliveries!
prevent a crime from taking place!!
deliver a fan's letter to their idol!
deliver an anonymous love letter <3
sample all the rare mushrooms!
do your part & clean up the forest!
glide for 5 minutes!
find all rare coins!
complete 25 deliveries!
find 20 trinkets!
complete 10 deliveries!
deliver a letter in under 30 seconds!
take over 15 minutes to deliver a letter!
file an official letter of complaint >:(
glide for 60 seconds!
find 5 trinkets!
get to know your neighbourhood
Deliver your very first letter!