An automated mail tray filling apparatus for taking envelopes from a vertical stacker output of an inserter machine and placing them in mail trays. A vertical stack of envelopes rests on a long stacker table. The mail tray filling apparatus has a track positioned next the vertical stacker table parallel to the length of the vertical stacker table. A mail tray transport is positioned beneath the track and arranged to transport mail trays beneath the track in a direction parallel to the vertical stacker table. A movable mail tray filler is movably mounted on the track to travel in parallel next to the vertical stacker table and above the mail tray transport, and is arranged to withdraw envelopes from the end of the vertical stack in a sideways direction. Envelopes are then redirected into a downward direction and fed into a mail tray positioned beneath the movable mail tray filler.