To change this behavior and always open the message in the Print Layout in edit mode:
Word 2013
Open Word-> File-> Options-> section General-> Open e-mail attachments and other uneditable files in reading view
Word 2010
Open Word-> File-> Options-> section General-> Open e-mail attachments in Full Screen Reading view
Word 2007
Open Word-> Office Logo-> Word Options…-> section Popular-> Open e-mail attachments in Full Screen Reading view
Word 2003
Open Word-> Tools-> Options-> tab General-> Allow starting in Reading Layout