Mail is divided into tabs including personal, House, event, system, battle, rebel army, resource, and favorites, their usages are as follows:
Personal: Includes mail from players and daily House chats.
House: Includes House related system notifications and House mail.
Event: Sending and claiming of event rewards.
System: Includes maintenance notifications, official mail and more.
Battle: Battle reports for individual battles and rally battles can be viewed here.
Rebel Army: Battle reports for Rebel Armies and Exile Fortresses on the World Map.
Resources: Mail for resource items obtained during World Map gathering.
Favorites: You can favorite mail, and it will be saved in this tab.