We don't offer a feature to forward receipts via e-mail at the moment. Our developers are considering it for a future update, depending on the number of users requesting such a feature.
If you would like to request this as a feature, please email us at [email protected] with the subject line: Feature Request - Forward Receipts via Email
In the interim, your best bet would be to simply take a screenshot of the email receipt on your phone, and then add the screenshot as your receipt image from your images gallery.