Most commonly used shortcut keys in E-mail
Shortcuts for a new message:
Ctrl+O - Open a message (or Enter)
Ctrl+N - Create a new message (in the Mail view)
Ctrl+Shift+M - Create a new message from anywhere in Outlook
Navigation shortcut keys:
Ctrl+Shift+I - Switch to Inbox
Ctrl+Shift+O - Switch to Outbox
Ctrl+Shift+B - Open address book
Ctrl+Y - Open a folder list
Ctrl+. (period) - Read next message (with a message open)
Ctrl+, (comma) - Read previous message (with a message open)
F6 - Read previous message (with a message open)
Shortcut keys for changing messages:
Ctrl+D - Delete an open item
Ctrl+Q - Mark as read
Ctrl+U - Mark as unread
F4 - Find or replace (with a message open)
Shift+F4 - Find next (with a message open)
Ctrl+P - Print
Ctrl+Alt+M - Mark for download
Ctrl+Alt+U - Clear Mark for Download
Ctrl+Alt+J - Mark as not junk mail
Insert - Toggle follow-up flag
Tools:
F9 - Check for new mail
Ctrl+B - Display send/receive progress
Alt+S - Send mail
Ctrl+R - Reply to the selected message
Ctrl+Shift+R - Reply all
Ctrl+F - Forward the selected mail
Ctrl+Alt+F - Forward as attachment
See also this tip in French: Les plus utilisées raccourcis claviers pour les courriels.