Mail messages frequently contain numerous headers, most of which are of interest only to the underlying sendmail program. For this reason, Ishmail gives you the choice of specifying which headers should be displayed and which headers should not be displayed (that is, should be hidden). (See Displayed Headers Option for information about specifying headers to be displayed.)
If you have used the Displayed headers ... option to hide certain headers, you can cause them to be displayed, on a message-by-message basis in the Reading window by clicking the Reading window's Show all headers option.
You can scroll the headers subwindow to view all of the headers, or you can use the resize button between the header area and the message body area to expand the header display area.