A player has it’s own private mail. You will receive artifacts after buying them (in auction house or premium shop) in your mail.
A player can take objects from mail and put them into a bag or a chest.
The mail is free.
You can find mail sign in the chest panel in a shape of an envelope.
There is no limit for slots in mail.
Artefacts and goods are kept in mail for 90 days. If a player don’t take them, they will disappear.