There's an extra storage for Oil and Coin, any oil/coin claimed from mail go there instead of going to your normal oil/coin pool, and the storage can store up to 600.000 oil and 1.8 million coins, you can withdraw oil/coin from there anytime
Here is the thing. Players can get "ressources" as "gifts" from random mail rewards from devs = "Game is now X years old. Here XXXX ressources." (you claim it or can let the mail/gift unopened in mail box)
So basically kinda, an exploit. To keep mail dusting in box. (its like letting the ammo truck in the road during the war, near the base)
Update is about limiting "mail box" (hence the reward mails)
It would be like (with truck example) : not allowing more than 100 parking places for the ammo trucks outside your base.
The only good news, they didnt lower it to 50 or 20. Nor deleted mails (i guess/hope so)
But problem is still is : No new ammo truck until you have less than 100 trucks parked.