If it's intentionally shielded from prying eyes, and possibly nefarious, it's the dark web. More often than not this would be on Tor, I2P, or similar, but that's not required. If it simply can't be easily indexed by search engines, it's the deep web. Overlap between the two is possible.
Your mailbox would be deep web, if it's exposed to the Internet. If your mailbox uses an auto-responder to respond to coded messages with encrypted binaries of an illegal nature, it would be dark web.
Your mailbox would be deep web, if it's exposed to the Internet. If your mailbox uses an auto-responder to respond to coded messages with encrypted binaries of an illegal nature, it would be dark web.