Since 1.09 I believe.
Yes, AI defend lose win rewards activate every 5 wins as of 1.0.9.
Oh i thought it was for the ai succeeding in defending my kingdom.
I knew about that bar and have filled it multiple times. it is just the first time i got mail about kingdom defends.
That’s odd. Sounds like you’re owed a whole ton of resources, then…
I am not mad. At least now i know the mail thing has multiple purposes…except sending mail to other users.
So… Why exactly did you think a lot of players set their defense team to a one troop deck? 
Well i had assumed for this reason actually but i did not think it would use my mail box as this is the first time getting it.
Fair enough, before 1.0.9 wich introduced the mail box, we did receive the same kind of rewards, but we directly got them at the end of the defense battle (wich makes one wonder why the mail box was introduced, it actually makes gathering rewards slower than before ._.) 
Because of top level guilds, to solve the problem of multiple number of quests completed simultaneously. Before mailbox a player could stack up to 20 uncollected rewards.
Strange limit indeed, but I believe they could have come up with a solution to extend that limit without a mailbox.
Mailbox makes collecting daily reward longer (and one may miss it too !)
Mailbox makes getting reward from defense longer and less intuitive
As for Guild tasks, mailbox didn’t make much difference in the way you collect it, just in the way you can stack them I guess.
But now, you go through the mail menu instead of the guild menu, making most people not even use the guild menu anymore (except when spending on guild tasks), so, not checking tokens collected by other guild members and not checking anouncement in “activity” tab
Kind of a shame imo 