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