Abstract: An anti-theft mailbox insert secured in a conventional mailbox for limiting access to delivered mail. The security box has an inclined front door that pivots open horizontally for retrieving delivered mail. When the door is closed, a gap between the door and the top panel of the insert allows mail to be delivered into the security box. The bottom portion of the door possesses a safety clasp that may be locked for securing the door closed. Outgoing mail is placed within the mailbox, resting above the top panel of the security box, for outgoing mail pickup.