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