One of the features Microsoft introduced into Outlook is to reply in the READING PANE rather than in a separate window. While I appreciate that this cleans up the interface, it does make navigating into old messages while composing new messages, quite messy.
In the Online version Outlook Web App OWA, you can fix this by clicking the POP OUT icon. The question is, in the desktop version, how can Outlook be set to use a new window for composing replies?
There are three answers, all of which are really the getting at the same change:
Outlook GUI to Use New Window for Reply:
FILE > OPTIONS > MAIL > REPLIES & FORWARDS > OPEN REPLIES AND FORWARDS IN A NEW WINDOW = enabled
Registry GUI to Use New Window for Reply:
HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\Message\disablereadingpanecompose = 1 (Hex value)
USER CONFIGURATION > POLICIES > ADMINISTRATIVE TEMPLATES > MICROSOFT OUTLOOK 2016 > OUTLOOK OPTIONS > COMPOSE MESSAGE > DISABLE READING PANE COMPOSE = enabled