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