There should be a protocol for amending emails. Say I sent an email, but later got new or extra information about the topic I had sent. I could send an amendment email that starts with AM: [email subject] instead of writing a whole new email. That way the recipient would have the context, and I can be lazy and not have to write as much.
Then again, I just wrote this blog post in the same time I could have written the email that prompted me to have the idea that prompted me to write this blog post.