Action Mac shortcut Windows shortcut
Archive a message Command–Down Arrow Control–Down Arrow
Bold Command-B Control-B
Compose a new email Option-Shift-N Alt-Shift-N
Delete a selected message or folder Delete Backspace
Flag or unflag email Shift-Command-L Control-Shift-L
Indent items in a list Command–Right Bracket Control–Right Bracket
Italic Command-I Control-I
Make font size bigger Command–Equal Sign Control–Equal Sign
Make font size smaller Command–Minus Sign Control–Minus Sign
Mark email as Junk or Not Junk Shift-Command-J Control-Shift-J
Mark email as Read or Unread Shift-Command-U Control-Shift-U
Open the selected message in a new window Return Enter
Outdent items in a list Command–Left Bracket Control–Left Bracket
Print Command-P Control-P
Redo changes to a message Shift-Command-Z Control-Y
Reply to sender Shift-Command-R Control-Shift-R
Save as a draft Command-S Control-S
Scroll through the selected messages Up or down arrows Up or down arrows
Select all messages Command-A Control-A
Send email Shift-Command-D Control-Shift-D
Toggle between Reply and Reply All Shift-Command-R Control-Shift-R
Underline Command-U Control-U
Undo changes to a message Command-Z Control-Z
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Note: Keyboard shortcuts may not be available in all countries and regions.

Source: Apple