Well, if you just want to add some line breaks, that is possible, as evidenced in this answer:
MailTo with HTML body
Relevant part:
If you are able to use javascript then "encodeURIComponent()" might be
of use like below...
var formattedBody = "FirstLine \n Second Line \n Third Line";
var mailToLink = "mailto:x@y.com?body=" +
encodeURIComponent(formattedBody);
window.location.href = mailToLink;
Otherwise, if you want a solution with the full monty (images, tables, ...) in y our e-mail, then you're out of luck with a simple "mailto" link.
Then you'll have to think of a way to construct the e-mail via ASP.NET and send it using the System.Net.Mail
namespace. Perhaps you can use ASP.NET to load in an HTML template of your e-mail, set some fields to the right text (with String.Replace for example) and then allow the user to click the send button (still in the browser) or send it automatically if there is no user action required.