Mailboxes need a Courier or Dark Messenger cast to function. They're both the same spell and function to different deities in the vanilla game. On Mythmoor all deities get the same spells and the two spells are duplicates. Once you cast one or the other on a mailbox it must be the same spell to improve the cast.

The mail system was never designed for sending hundreds of items at once or mass sending of items. Doing so can and will break your mail. You will have to wait until the mail times out and is returned back to you. Sometimes the mail is lost.

Limit what you send to 100 items at a time and wait until the player collects the items from the mail system before sending another 100 items.

$101-Cast=Time$