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