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