Typing SPC during a message preview exposes the body of the
message. If the message was new or previously unread, it will be flagged
"read". At this point you can use SPC to scroll forward, and
b or DEL to scroll backward a windowful of text at a time.
Typing space at the end of a message moves you to the next message. If
the value of vm-auto-next-message is nil, SPC will
not move to the next message; you must type n explicitly.
If the value of vm-honor-page-delimiters is non-nil, VM
will recognize and honor page delimiters. This means that when you
scroll through a document, VM will display text only up to the next page
delimiter. Text ater the delimiter will be hidden until you type
another SPC, at which point the text preceding the delimiter will
becom hidden. The Emacs variable page-delimiter determines what
VM will consider to be a page delimiter.
You can "unread" a message (so to speak) by typing U
(vm-unread-message). The current message will be flagged unread.