The Ebony Mail is arguably the most powerful cuirass in Morrowind. It is medium-class armor, weighs 20, and has a base AR of 100, same as the highest heavy armors. Additionally, it has a constant effect enchantment with 75% resistance to fire, 20% resistance to regular magic, and 50 shield(adds 50 to your total armor rating).
The normal way to get the Ebony mail is to play all the way through the Temple faction. However, due to an oversight, you can do the Four Corners and Mount Assarnibibi pilgrimmages at any point, even if you haven't joined the temple. Here's how:
1.Get four Daedra hearts, then go to Dagon Fel. Head west out of Dagon Fel, until you get to an intersection, then head south. The next intersection go west. After you see an Orc, go a little further west until you find a path going north. Fight or sneak past the hostile orcs on this path, then activate* the shrine of Malacath. Don't drop the daedra hearts, keep them in your inventory and the game will take care of the donation. Click here for more info, and a map.
2. Got to Ald Sotha, the daedric ruins northeast of Vivec City, and kill everyone inside, then activate* the shrine.(all enemies inside must be killed to activate this shrine)
3. Go inside Bal Ur, just north of Suran. Fight or sneak past the orcs, and activate* the shrine.
4. Go to Ald Daedroth, a Daedric ruin on a large island east of Dagon Fel. Fight/sneak your way deep inside, and find a friendly Orc. Give him some moon sugar, and he'll give you a good hint to find GambolPuddy. Pick it up, then find and activate* Sheogorath's statue to donate Gambolpuddy. Click here for more.
5. Finally, go activate* the shrine on Mount Assarnibibi, which is north of Molag Mar. You instantly recieve the Ebony Mail (as long as you have correctly completed the other 4 pilgrimmages I mentioned.)
*to activate a shrine: get close and face the shrine statue, and press your talk/open door button.
If you later decide you want to do quests for the temple, there won't be any problem. When you reach the end of the temple quests, just take the Ebony Mail with to show him you got it.
Notes: I was not the one that discovered the shortcut existed, but I used the game editor to find out why and how to do it. This was originally used to settle an argument on the Bethesda forums.