Thats true. Same goes for the armory which can get that sort of data and update it as need be.

But for things like Mail you need to log in. Do ya'll remember when they introduce the new transmog system which meant that completed quests and their rewards was now available for transmogs? To get all the transmogs, you had to log in to each character at least once and get that data from that character.

Same goes for this prepatch when they merged the pvp levels. You needed to log in to each of your characters to get a fully up-to-date merged Honor level.

Its those sorts of things Im talking about with the mail. Now I dont know if Blizzard could make it so that mail is an easily accessible thing even out of game, or at least without logging in directly but as it stands, the status of mail on a specific character is not updated until youve logged on to that character.