The Message Composition Window is shown below.

Opening
–Main Menu > Message > New, Reply, Reply to All, Reply to List, Forward, Forward As, Redirect, or Edit As New Message.
Mail:
–Folder Pane > New Message.
–Message List > Message context menu > Reply to Sender Only, Reply to All, Reply to List, Forward, Forward As, Redirect or Edit As New Message.
–Message View Area > Recipient context menu > Compose message for.
–Calendar Pane > Event or task context menu > Convert To > Message.
–Today Pane > Event or task context menu > Convert To > Message.
–Event List > Event context menu > Convert To > Message.
–Event Creation Window > Event > New > Message.
–Contact List > Contact context menu > Write.
Hotkeys: Ctrl+N or Cmd+N.