Normally, mail programs only displays a few email headers, such as the
"From:", "Subject:",
and "Date:" fields.
For the purpose of configuring Procmail or
SpamAssassin you will often want to inspect
other headers, like "X-Spam-Status:".
In Elm and Mutt, you do this by pressing "h" in either the
index or pager.
In Pine, you also use "h", but if you get a message like
[Command "H" not defined for this screen. Use ? for help], -
you may have go to (M)ain, (S)etup, (C)onfig and check the
"enable-full-header-cmd" in the
[ Advanced Command Preferences ] section.
References: