Abstract: An envelope sealer includes a first horizontal plate for receiving an envelope, a vacuum manifold for drawing open the envelope's downward-facing flap as it departs from the first horizontal surface, and a plow for catching the flap and directing it such that the flap travels below a second horizontal plate while the body of the envelope travels above the second horizontal plate. A fiber-optic sensor system is provided for detecting the non-presence of a flap during the travel of the envelope over the second horizontal plate and for actuating a downstream diverter in response thereto. An unseal unseal ramp is provided for lifting the flap over the plow if it is determined that the envelope should not be sealed. During the flap 's travel below the second horizontal plate, it passes and engages a wetted brush, whereby water is applied to the flap's adhesive surface.