Rune Factory uses "Order Signs" (Prince Points) rather than mailbox passwords. If you are trying to unlock something and it isn't working:
Good news: The passwords have just been updated! farm rpg mailbox passwords updated
Farm RPG has a delicate player-driven economy. If early players could stack infinite gold from perma-codes, new players would face massive inflation in the player market (the Exchange). By expiring codes every 3-6 weeks, the developer ensures that free rewards act as a "boost" rather than a currency printer. Rune Factory uses "Order Signs" (Prince Points) rather