I would like to send a Thank-You card, but I don't have the email address of the sender.
Underneath the ecard that you have received you will notice the fields “From” and “To”. Next to these fields are the names of the sender and your own, respectively. By clicking on the name of the sender, an email will pop up with the email address pre-filled. Alternatively, you can also see the email address of the sender by pointing to the sender's name with your cursor and the email address will appear at the bottom of your browser window.