Get New Mail: Cmd-Shift-N
If you’re waiting for a message, this can be really handy. Cmd-Tab to Mail, hit Cmd-Shift-N, then Cmd-Tab back to your previous app.
Reply: Cmd-R and Forward: Cmd-Shift-F
These pretty much go together. Cmd-R will reply to your message and Cmd-Shift-F will forward your message. Always handy if you have a lot of messages to reply to.
Mark as Junk Mail: Cmd-Shift-J
No matter how hard you try, you will probably never be completely free of Junk mail. However, if you mark a junk message as such, then Mail will eventually learn what you want and what you don’t want.
Send Message: Cmd-Shift-D
On that same note, once you’ve hit Cmd-R to reply to a message, you can use Cmd-Shift-D to send the current message. This can greatly decrease the time spent dealing with emails.
Search Your Mailboxes: Cmd-Option-F
This is probably one of the one’s I use the least. However, that’s probably because it’s a strange shortcut. So, I’ve changed mine to Cmd-K. If you do this, you’ll need to change the shortcut for Mailbox>Erase Deleted Messages>In All Mailboxes. I changed that to Cmd-Option-Ctrl-K.
Quicklook Attachments: Cmd-Y
While you can use Cmd-Y in Finder, you cannot use Cmd-Option-Y (Fullscreen quicklook) in mail. Bummer. (By the way, Cmd-Option-Y is for New Todo.)
New Viewer Window: Cmd-Option-N
This one is good to know simply because all too often I’ll close the main viewer and need to go looking through the menus for the New View Window option. (Of course, it doesn’t occur to me while I’m looking that I can simply click on the dock icon.)
Increase/Decrease Quote Level: Cmd-‘ and Cmd-Option-‘
This one I only use every now and then, but it’s still helpful. As Omer over at MacTips.org points out, a good way to remember this is that you’re just pressing Command and the key you use to signify quotes in a normal document.
Bounce Messae: Cmd-Shift-B
This goes back to the Junk Mail prevention department. Supposedly, it helps stop spammers if you bounce the messages they send you. (It will appear to them as if the message was sent to a bad address.)
New Mail Message: Cmd-N
This is probably the most obvious of all of these. But you should be using Cmd-N to make a new message and not be clicking in the toolbar. (Or even worse, the menu bar.)