It is considered good etiquette not to quote email without permission. However, these days, emails are often part of a broader discussion spanning blogs, web fora, and so on. It's increasingly easy to run afoul of this etiquette rule. Conversely, it seems awkward to say something like "permission granted to quote" when I respond to, say, a blog entry in an email.
Thus, I propose the two-character string "+ as a shorthand indicating that permission to quote, with attribution, is granted. Permission is also granted to integrate the email into any copylefted documentation (most copylefts do not require attribution).
Analogously, "- is a request that the email not be quoted, perhaps because it contains confidential or sensitive information. Lastly, when someone is known to be familiar with the convention, "? asks whether quoting is permitted.
Spread the meme! "+