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