Most of us have, at some point in our digital lives, experienced email regret—a sinking feeling that comes after you hit the send button in anger, or following one too many glasses of wine.

Two Harvard law school students have an answer to that: emails that expire once read. The idea is to recognize that not every email needs to be around forever, say David Gobaud and Lindsay Lin, who have introduced a free Web-based email service called Pluto Mail.

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