Previewing is VM's way of showing you a small portion of a message and allowing you to decide whether you want to read it. Typing SPC exposes the body of the message, and from there you can repeatedly type SPC to page through the message.
By default the sender, recipient, subject and date headers are shown
when previewing; the rest of the message is hidden. This behavior may
be altered by changing the settings of two variables:
vm-visible-headers, vm-invisible-header-regexp and
vm-preview-lines.
The value of vm-preview-lines should be a number that tells VM
how many lines of the text of the message should be visible. The default
value of this variable is 0. If vm-preview-lines is nil,
then previewing is not done at all; when a message is first presented it
is immediately exposed in its entirety and is flagged as read.
The value of vm-visible-headers should be a list of regular
expressions matching the beginnings of headers that should be made
visible when a message is presented. The regexps should be listed in
the preferred presentation order of the headers they match.
If non-nil, the variable vm-invisible-header-regexp
specifies what headers should not be displayed. Its value should
be a string containing a regular expression that matches all headers you
do not want to see. Setting this variable non-nil implies that
you want to see all headers not matched by it; therefore the value of
vm-visible-headers is only used to determine the order of the
visible headers in this case. Headers not matched by
vm-invisible-header-regexp or vm-visible-headers are
displayed last.
If you change the value of either vm-visible-headers or
vm-invisible-header-regexp is the middle of a VM sesssion the
effects will not be immediate. You will need to use the command
vm-discard-cached-data on each message (bound to j by
default) to force VM rearrange the message headers. A good way to do
this is to mark all the messages in the folder and apply
vm-discard-cached-data to the marked messages. See section Message Marks.
Another variable of interest is vm-highlighted-header-regexp. The
value of this variable should be a single regular expression that
matches the beginnings of any header that should be presented in inverse
video when previewing. For example, a value of `"^From\\|^Subject"'
causes the From and Subject headers to be highlighted.
By default VM previews all messages, even if they have already been read.
To have VM preview only those messages that have not been read, set the
value of vm-preview-read-messages to nil.
Typing t (vm-expose-hidden-headers) causes VM toggle
between exposing and hiding headers that would ordinarily be hidden.