Death Knight Quests, lore and player interaction

Q: Why can all races be Death Knights and why are you accepted by your previous factions.
A: You fell during a battle against the Scourge and were raised with the power of necromancy to serve as the spearhead on the Lich King's assault against the Light's Hope Chapel. Without getting into too many spoilers, after the assault you are free of the Lich King's grasp and rally under your factions leader to attack him.
Credit to Tacitus

Q: Can Death Knights obtain mail before leaving Acherus?
A: Yes, a Death Knight may obtain mail at an early stage in their quest chain yet there is only one mail box which is constantly surrounded by hostile crusaders.

Q: Can a Death Knight join a guild before leaving Acherus?
A: Yes a Death Knight may join a guild before finishing their initial quest chain, please bare in mind that you will NOT encounter a charter vendor untill finishing.