Why charge gold to sned messages / items?

I just thought of this now.. but why is the charge even there? If anything it deters people from sending messages. Is there some sort of giant AOEO bank vault and it needs money deposited from stores and mail to be able to send out coin for rewards? xD So what is the reason anyway?
|||its called a money sink, most mmos have them. they are desisned to help deflate the economy a little bit. different mmos use different money sinks but most use mail fees as one.
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also, yes the fee is designed to deter people from sending mail, without the fee people would send pointless stupid and useless mails and clog the mail system up.

(sorry for the doublepost, cant find the edit post option)