Yes, you can mail it.

The other day | visited the post office to buy some stamps. While in line, a young woman came up behind me. She had an envelope in her hand, so I suggested she go before me. When she got to the counter, she asked the bored clerk whether she could place the envelope in a blue mailbox. The clerk replied, "It has a stamp on it so yes you can. You can even put it in a mail slot, a mail drop, or even a mailbox."

The woman confirmed: "So I can put it in a blue mailbox?"

The clerk assured her that was the case, so she left, presumably to put it in one of the blue mailboxes outside. 

It was an odd exchange, so I chuckled to myself; however, I realized that the girl had quite possibly never mailed anything before in her life: She was probably 20 or so and thus did everything on her phone or online. 

lt made me feel rather dated.