MailCOPA includes two viewers for messages:
The Quick Viewer (see here for more details), which forms part of the Main Window, unless you have configured MailCOPA to hide this viewer in the General Preferences.
The Main Viewer, which forms a separate window for viewing a message. Multiple windows can be opened for different messages and placed anywhere you want on your display. The last instance of the Main Viewer to be closed will remember its position on the display, and new instances will be placed in this position.
There is also the Image Viewer, accessible from both of the above, which provides a quick view of attached .gif and .jpg images. See Attachments for further details.