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!
sample all the rare mushrooms!
meet everyone in the neighbourhood!
find 40 trinkets!
complete 50 deliveries!
deliver a fan's letter to their idol!
deliver an anonymous love letter <3
do your part & clean up the forest!
glide for 5 minutes!
find 20 trinkets!
complete 25 deliveries!
prevent a crime from taking place!!
deliver a letter in under 30 seconds!
take over 15 minutes to deliver a letter!
glide for 60 seconds!
complete 10 deliveries!
file an official letter of complaint >:(
find all rare coins!
get to know your neighbourhood
find 5 trinkets!
Deliver your very first letter!