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.
deliver meals instead of mail!
you did it!! kinda... not really.... BUT you did do other things!
find 40 trinkets!
glide for 15 minutes!
deliver a fan's letter to their idol!
find all rare coins!
complete 50 deliveries!
sample all the rare mushrooms!
do your part & clean up the forest!
prevent a crime from taking place!!
deliver an anonymous love letter <3
meet everyone in the neighbourhood!
complete 25 deliveries!
find 20 trinkets!
take over 15 minutes to deliver a letter!
glide for 5 minutes!
complete 10 deliveries!
deliver a letter in under 30 seconds!
glide for 60 seconds!
find 5 trinkets!
get to know your neighbourhood
file an official letter of complaint >:(
Deliver your very first letter!