How can I send a pre-composed email to a Gmail user, for them to edit and send?

I want to:

• Compose an email A
• Send it as an attachment in another message B to Gmail user X

User X having received email A wants to:

• Open email A
• Edit it
• Send it (From: user X)

How can this be done?

I tried sending a EML but Gmail does not allow user X to edit or send it.