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!
sample all the rare mushrooms!
find 40 trinkets!
deliver meals instead of mail!
deliver a fan's letter to their idol!
complete 50 deliveries!
glide for 5 minutes!
meet everyone in the neighbourhood!
prevent a crime from taking place!!
deliver an anonymous love letter <3
do your part & clean up the forest!
complete 25 deliveries!
find 20 trinkets!
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!