When I need to be sarcastic in emails, I usually don't end up sending those emails. If I do, I might add a :-P, or for a more meta approach, "I really can't stand you," or maybe just the old standby, "just kidding!" Which, when you think about it, and who does anymore really, is so sarcastic sounding that it probably needs its own "just kidding!" Just kidding.
Thank goodness then that a company called Sarcasm, Inc, has invented a brand new writing technology to address this international crisis in ironic interpretation: a punctuation mark for sarcasm. Based in Michigan, the American capital of this brand of humor, Sarcasm, Inc. has likely spent years developing its innovation – something like an @ symbol, but upside down, and with a dot in place of the a — and probably another decade just waiting for the Internet to pick up on it, because it was more important with other things. And also because the world didn't really get sarcasm until recently. Fortunately Jay Leno's back on the Tonight Show. (It's worth noting here that the SarcMark may have been beaten to the punch by the percontation point (1580) and irony mark (late 19th c). Neither caught on.)
This raises another important point, and I don't mean the exclamation one. I thought the essence of sarcasm — which Thomas Carlyle once hilariously called "the language of the devil, for which reason I have long since so good as denounced it" — was the dry, unemotive tone that accompanied the words. If we do start using the SarcMark, it should make writing and especially Tweeting easier. But what will befall our spoken sarcasm? Will lazy sarcastics just start inserting the SarcMark into their speech? "Loved it! (SarcMark)" That might be a real gut-buster. (There's even a bit of software, though good luck figuring out how to download it.)
And then again, will we even really need a SarcMark when we're all communicating by video phone, or using some kind of Emotion Markup Language, or by sending each other youtube links, or corporate logos? What we'll need then is not something to denote sarcasm, but something to indicate that the symbols we're using don't belong to us, but to our friends, the big corporations. Maybe we should call it the TradeMark™.
I am just kidding.