Here's how you can tell Siri to write an email reply on your iPhone.
1. Open the Mail app.
2. Open the email you want to read.
3. Long press the Home button to call up Siri.
4. When Siri loads, say the word "reply."
5. Siri will ask you what you want the email to say.
6. Speak your response.
7. Siri will show you your reply.
8. Say "send" or "cancel" and Siri will send or cancel your reply.
For more, see the original article at the link below.
iPhone 5 Tips and Tricks | Apple