Abstract: The email attachment security system and method using out-of-band authentication allows a recipient to receive a secure digital file from a sender. The system server receives from the sender a send request, a digital file with a recipient's email address and phone number. The system server encrypts the digital file and sends an email to the recipient with a clickable link that, when clicked, initiates the out-of-band authentication by opening a customized webpage requesting the recipient verify the sender-provided recipient phone number. After verification, an authentication PIN is forwarded to the recipient's telephone via voice or text message. This authentication PIN is input at the customized webpage by the recipient to complete the authentication. Then the encrypted digital file (attached to the email in the first aspect) is decrypted and access to the digital file is provided to the authenticated recipient.