Hello,
I notice that a lot of people think that they have a mail bug when in fact they are using 10 out of 10 messages. I think the problem stems from the fact that there is no clear “Hey, your inbox is full!” indicator.
I’d suggest a system similar to when your inventory gets full… the icon for the mailbox should turn red when the mailbox is full. That way you’d have something that draws your attention and you’ll know that you need to clean out the inbox to make room for stuff your friends might send you.