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