I jumped on the PTR to try and find the letter people have been talking about, and I was successful! Here are some screenshots. Ignore the almost dead Skimmer; I hadn’t changed teams after killing Nefarious Terry.
Basically, you click on the letter and it will despawn, and nothing will come up in your bags (at least it didn’t on the PTR). After you click on the letter, head back out of the Slaughterhouse, like you’re heading back to Terry, and make a left instead.
The clickable mailbox will be in the little alcove. Once you click on it Postmaster Malown will spawn.
Plagued Mailemental (Magic)
Soul Collector (Undead)
His fight is way easier than Fras. I used a Mountain Cottontail (H/S, 2,2,2) in slot #1, because his health stands up to the attacks, he has a dodge for rounds 1 & 2 to avoid Agony & Curse of Doom, and he’s faster than the Mailemental (311 speed). Then a Dreadmaw (2,1,2) in slot #2. The strat is up on Xu-Fu’s if you want the step-by-step.
Best of luck!