Lost Mail

Tips:

1. 
   
2. If you loot Lost Mail from near the mailbox by the Violet Citadel now patched to be from "a random location near one of several Dalaran mailboxes", it is BOP. But at the end of the quest line, you get the Lost Mail item again as a BOE! This means that:
   
   • As of 21 January 2018, the title is NOT account-wide. So first complete the quest chain on whatever character you looted it with. Then mail the BOE Lost Mail to the character you want on, and complete it, mail it, etc. until you're done with all characters. Now patched to be account-wide!
   • Easier than camping a mailbox 24/7 is: you can just start the quest by "renting" Lost Mail from the AH--buy it, quest through it, then sell the item again when you get it back.
3. 
   
   
4. There is no direct follow up quest from [npc=100986] (or anyone beside her) after you turn in the Lost Mail. The next step is receiving mail from the Postmaster inviting you to help him out. Check your mail to read it, then fly behind the Magus Commerce Exchange (/way 32.8, 32.1 in Dalaran). There is a [object=247848] that looks like a brown pot within a neat triangle-shaped wall that you click to visit the Postmaster, sort of like Mario going down a pipe. It looks like this: https://imgur.com/a/lGX2R
5. 
   
6. Doing the questline gives you the [item=156833] toy. Doing it also unlocks a mini-game wherein you can then work on completing 2 achievements: Priority Mail and Post Haste. Doing these will get you the [item=156721] pet and the [title=521] title.
7. 
   
8. Doing the Priority Mail achievement will also complete Post Haste. So just go straight to sorting 30 incomplete addresses when the time comes; no need to work on sorting 30 complete addresses separately.
9. 
   
10. Use this weak aura (all credits to the author, it's not mine) when doing the incomplete mail achieve: https://wago.io/HkU2aa1Sf
11. 
    
    

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Patch notes source: https://worldofwarcraft.com/en-us/news/21365424#1

FWIW, When I went to turn in this quest to Madam Goya on my druid, she had the yellow ? over her head, but clicking on her did nothing. Turns out she wouldn't talk to me in Cat from. Once I exited to normal form, she accepted the turn-in.

I used a class trial to get the mobile mailbox toy: Katy's Stampwhistle. I bought the Lost Mail item for 4k on a cheep server.