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