If a knock trigered an email, you would have to put some kind of limit, like it wont send another email for 5 minutes, or something like that.
"DING.....You have 958 new messages"
I'm thinking more along the lines of, say someone needs physical access to my office, they could call or email me directly. I would then send an email to an address accessible by the Arduino telling it that I want to activate the door opening mode. The program would then send back to me a combination which I would then give to the person so that they can enter the combination and the door will open, but the combination would work only that one time. This could be done with a keypad, but I think it would be neater if it could detect a pattern being knocked on the door.