It’s actually very easy once you get the Lost Mail. If you want this title, be sure to check at the foot of every Dalaran mail box for a tiny little red-stamped letter. After that, it’s pretty easy especially if you are druid and have the dreamgrove portals.
Note: it is Broken Isles Dalaran ONLY. If you want to check out where the entrance to the Postmaster’s Office is beforehand, find the tube with the little letters zooming down, near the pet graveyard.
Grats!!!
(cookie)
Well done!
Awesome sauce! Congratulations! 
Like I said, it is very easy to get it once you find the Lost Mail. It’s no big accomplishment I just happened to get lucky.
If you don’t have the patience for that, you can check the AH. After completing the quest line, you get another Lost Mail that can be traded or sold.
Congratulations!
Gratz!! Now bring me my mail.
Congratulations! Now I know that my mail containing valuable goods, such as gold, minions, pets and explicit contents will now be delivered on time and with an unbroken seal.
You’ve been Malowned! Postmaster . . .
My mail was late again. Still waiting on my package. 