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