Accessed from Configuration 
Preferences Messages Viewing Headers
This allows the setting of headers to be displayed in the Information Panel (see below) and on message print-outs. Any header(s) present in the message may be displayed or printed, even non-standard ones (eg X-Env-Sender).
Click on a Hot Spot on the image below:
This is displayed as part of the Quick Viewer:
The headers displayed in the Information Panel can also be configured by right-clicking on it and selecting Customise Headers from the context menu.
Other Print settings are here.
The settings for the information panel can be accessed from the Quick Viewer and separate Viewer