That may be tougher, I am not sure if InfoPath has a set format of the email or if it is determined by the defualt for each Outlook client. Whether it is HTML, or plain or rich text. You could experiment with some html characters to try and see if you can do it but it may not be consistent.
To me it is easier for them just to open the form.
With the Database Accelerator and its workflow you can have a custom email that is sent from the server which is sent automatically and could be formatted more like what you are attempting. It also will send notification when there is something overdue or late. Just FYI