Zip+4 (9 digit) can't ever be more than 100 mailboxes because 11 digit is guaranteed unique. It follows that a 5 digit zip is limited to 1M boxes. Unless I hav

Zip+4 (9 digit) can't ever be more than 100 mailboxes because 11 digit is guaranteed unique. It follows that a 5 digit zip is limited to 1M boxes. Unless I have a misunderstanding about 11 digit being unique anyway.