I guess you mean 1 specific email message, right? You need to be careful because when you say "your email" I'm thinking "my whole inbox"

In general I think email is a very private thing (for the receiver, but there's also an implicit assumption of the sender)

However, I can imagine a few use cases. People tweeting copies of their acceptance letters to university: vanity. Also people could emial2tweet annoying marketing emails: public shaming companies. That would be nice---maybe then the recent email marketing craze will calm down!