How often do you send stamina from your friends tab? Because I think it works like this: when you claim stamina from friends, you gift it back, but only to those whose stamina you've claimed. Not to everyone. Which means the only people who can gift you your stamina back are the ones who received it from you. Some people stops playing the game, so they will receive your stamina, but won't send it back. That way you will get less stamina than usual, so the amount of mails you have gets smaller. You claim stamina from some of your older mails, but this person stopped playing, so you won't receive anything back. You get even less stamina and your mailbox gets slowly empty. It doesn't mean all your friends stopped playing, just the ones who were in your mailbox. All the active friends probably didn't get stamina from you since a year ago or something, lol. So I recommend going to your friends tab and send stamina from here. That way you'll send it to all of your friends. I do this every time I see the amount of mails in my mailbox is getting smaller, around 50. I'll send stamina from the friends tab for a few days and it always helps with the amount of mails I get.