At any point you can press ? to see a full list of keyboard bindings.

Here are some from different modes:

Help for thread-view-mode
=========================

Keybindings from thread-view-mode
---------------------------------

      h : Toggle detailed header
      H : Show full message header
      V : Show full message (raw form)
  Enter : Expand/collapse or activate item
      E : Expand/collapse all messages
      e : Edit draft
      y : Send draft
      l : Edit or add labels for a thread
      o : Expand/collapse all quotes in a message
      n : Jump to next open message
    C-n : Jump to next message and open
      p : Jump to previous open message
    C-p : Jump to previous message and open
      z : Align current message in buffer
      * : Star or unstar message
      N : Toggle unread/read status of message
      r : Reply to a message
      G : Reply to all participants of this message
      f : Forward a message or attachment
      i : Edit alias/nickname for a person
      s : Save message/attachment to disk
      A : Save all attachments to disk
      S : Search for messages from particular people
      m : Compose message to person
      | : Pipe message or attachment to a shell command
      a : Archive this thread, kill buffer, and view next
      d : Delete this thread, kill buffer, and view next
      w : Toggle wrapping of text
     .a : Archive this thread and kill buffer
     .d : Delete this thread and kill buffer
     .s : Mark this thread as spam and kill buffer
     .N : Mark this thread as unread and kill buffer
     .. : Just kill this buffer
     ,a : Archive this thread and view next
     ,d : Delete this thread, kill buffer, and view next
     ,s : Mark this thread as spam, kill buffer, and view next
     ,N : Mark this thread as unread, kill buffer, and view next
 ,n, ,, : Kill buffer, and view next
     ]a : Archive this thread, kill buffer, and view previous
     ]d : Delete this thread, kill buffer, and view previous
     ]s : Mark this thread as spam, kill buffer, and view previous
     ]N : Mark this thread as unread, kill buffer, and view previous
 ]n, ]] : Kill buffer, and view previous

Keybindings from line-cursor-mode
---------------------------------

     Down, j : Move cursor down one line
       Up, k : Move cursor up one line
       click : Click on item
 doubleclick : Double-click on item

Keybindings from scroll-mode
----------------------------

           J, C-e : Down one line
           K, C-y : Up one line
             Left : Left one column
            Right : Right one column
 PgDn, Space, C-f : Down one page
   PgUp, C-h, C-b : Up one page
              C-d : Down one half page
              C-u : Up one half page
       Home, ^, 1 : Jump to top
        End, $, 0 : Jump to bottom
                [ : Jump to the left
                / : Search in current buffer

Global keybindings
------------------
      q : Quit Sup, but ask first
      Q : Quit Sup immediately
      ? : Show help
      b : Switch to next buffer
      B : Switch to previous buffer
      x : Kill the current buffer
      ; : List all buffers
      C : List contacts
    C-l : Redraw screen
   \, F : Search all messages
      L : List labels
 P, ERR : Poll for new messages
   m, c : Compose new message