Brother MFC-8640D - Manual (Page 19)

USING THE MACHINE AS A PRINTER 1 - 12
4
Using both hands put the envelope in the manual feed tray until
the front edge of the envelope touches the paper feed roller.
Hold the envelope in this position until the machine
automatically feeds it in for a short distance, and then let go of
the envelope.
■ Make sure the envelope is straight as you insert it in the
manual feed tray. If it is not, the envelope may not be fed
properly, resulting in a skewed printout or a jam.
■ Do not put more than one envelope in the manual feed tray
at any one time, or it may cause a jam.
■ Put the envelope in the manual feed tray, making sure that
the side to be printed on is face up in the tray.