SendLetter fonksiyonu negatif değer alamaz.

MAIL_BOX_SEND proc. için dönen değerler düzenlendi.

Return values:
* 250 - You cannot send a letter to yourself.
* 251 - Exceeded the allowed limit for sending coins.
* 252 - There's not enough coins in the bag.
* 253 - You do not have enough money for postage.

* 254 - (Dialog)The receiver's mailbox is full.
Please try sending your letter at a time.

*If you buy a premium service,
you'll be able to receive up to

* 255 - Failed sending letter.