The first 480 lots will have mailboxes in the Mail Building located across the pool area from the Clubhouse. 

There is no direct numbering scheme with the mailbox number and the lot number.  To determine your mailbox number  is found in the Retama Lookup Table.

If you receive a package that will not fit in your mailbox, the postal clerk will put it in a larger box and leave the key with that box number in your mailbox.  When you retrieve your package from the larger box, just leave the key in the box lock.  The postal clerk will collect the keys in the next delivery.