In a mail inserter having an envelope movement mechanism to move an envelope into an insertion station and a feeder to move a pack of insert material into an insertion position so that the insert material can be inserted into the envelope, a linear array of optical sensing elements is used to determine the position of one edge of the insert material and another linear array of optical sensing elements is used to determine the position of one edge of the receiving envelope in order to make sure that there is sufficient end clearance between the insert material and the receiving envelope. A stepper motor is used to adjust the envelope position, if the end clearance is outside a predetermined range.