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