If you want to stick with javascript and posting the form to an e-mail address, your options are limited.
Here's a trick. Make two forms on your page - one with all of the fields, and a second with one hidden text field and a submit button. When you submit the second form, send it to a javascript function for "validation". In the validation function, set the value of the one hidden string field in the second form to include all of the values from the first form, formatted however you like. (Start it off with a "\n", or newline character for best results.) Then let this second form submit through to the e-mail posting.
Your e-mail message will contain one field value, the value of the hidden text field in the second form. Since you formatted the text in this field yourself, you can make it look pretty much however you like.
You do still get the "fieldname=" on the first line of the e-mail message, but the rest of the message is completely in your control.