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