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