The invention disclosed herein provides for automatically controlling positioning of a moistening nozzle relative to an envelope flap being moved therepast at high speed. The invention has particular application as an envelope flap moistener in a high speed mailing machine. Envelopes are moved to and past the nozzle at a speed of at least about 65 inches per second along a given path with the flap extending from the envelope in an opened configuration thereof. A sensor attached to the nozzle senses the location of the edge of the flap within about 0.2 inch of the nozzle upstream thereof a plurality times as the flap moves to and past the nozzle. The nozzle is then positioned to track the flap edge in accordance with sensing of the envelope flap. The nozzle may be pre-positioned by a lower bandwidth servo system having an envelope sensor located further upstream of the tracking sensor. A higher bandwidth servo loop controls tracking of the nozzle in response to the tracking sensor. It is not necessary to use stored or computed envelope flap profiles, envelope speed, etc., for the nozzle to track the envelope flap.