First thing I check when I get something like this is to do a cfdump of the form scope at the top of the action page followed by a cfabort. Will show you exactly what is in the FORM scope (and most importantly in this case, what is not). I often misspell the name of a form element in the submission page or on the action page, which causes this cheerful little fellow.

Because the form has a textbox in which u r entering the "from email address" but the textbox not named TXTFROMEMAIL . the mail application is still sending the mail because FROM option is is not mandatory. check the form and find out whats the name/id of the textbox where u r entering the FROM email address.