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