If there are no other restrictions on this topic, that is to say, there is no requirement to throw at most one letter in one mailbox, then each letter may be thrown into any of the six mailboxes, that is, 6x6x6. If you think in terms of mailboxes, it is more complicated, because each mailbox may contain six kinds of letters, and other mailboxes must be excluded, because it is impossible for every mailbox to have letters. So 5X5X5X5X5X5 has no reasonable mathematical explanation at all.