For what it's worth, you can't really respond by email to a PM, the rather cumbersome PM system here doesn't allow it. When you get a PM from someone, which comes to you in email, there should be a link to the senders's profile. Click that link, and it brings you to the PM 'page' for that user, where you type your message in and hit send. they'll get your PM in their email.
If you want their question in your PM, you have to copy and paste it into your PM.
As I said, ugly, cumbersome, and rather outmoded/outdated. But it is what it is.