Any type of mailbox refers to when you expect to receive news or communicate with others.
Mailboxes generally refer to an isolated state. American Metal Mailbox stands for Illegal Action. A letter to such a mailbox is a harbinger of inaccuracies or mistakes in your life.
Dreaming of a mailbox in the United States is primarily a precursor of hearing news from a distant friend. To dream that you want to drop a letter in a mailbox in the US but fail means you will not succeed is something you hear in waking life.
The US mailbox can have both positive and negative connotations in a dream, but generally, it refers to your expectations in waking life.
Surprised. Content. Surprise. Curious. Take advantage. Enjoy. Worried.