That has already happened to me.
Well, as you and me are registered and active users on this site currently, we are able receive letter here. That is normal.
If you have a membership, you can opt to send a letter to anyone or not. If you send a message to someone, then there are more two possibilities: either the destinatary will reply you (before or after the expiration of your membership) or not. That will depend on the fact if the letter was read or not, how active is the user, the interest of the respective member in you, the demand of contacts, presence of fake, account settings, blocked users, and other things. So, for several reasons, including the weird or bad intentional ones, the quantity of letters sent or received per period may be varied along the time.