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