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