I was thinking along the same lines Jules... creating an HTML template for use in the email.
Or create an image and embed that into the body of the email. Meaning the entire post card is an image. The problem with that is many people (myself included) have image viewing disabled in their email client for safety reasons. Not to mention the size of the image, in the end might be too big anyhow.
Also, it is important to note that if the client has a lot of contacts she wishes to send this email to... there is the possibility that his/her ISP may temporarily block or even shutdown her email as a safeguard against spam. This is one reason for using an email marketing service like Mailchimp (which is free), Constant Contact or something else. Just some additional food for thought (which I am pretty sure you knew already

).
Darlene