in reply to set REMOTE_ADDR to sendmail?
Your real problem seems to be the conversion rate of your process.
Have you considered shortening the steps to complete the form? Using AJAX and modern Web development techniques, you should be able to craft a form that will automatically adapt to the choices of the user and load different version of the form as the user fills it. This would shorten your funnel and result in a better conversion rate.
Also, if you really cannot shorten the process (and I really doubt that), you should make sure that the number of steps the user will go through are well indicated like Step 1/3, Step 2/3 etc. This could help.
Finally, if I understand correctly what you are trying to achieve with Sendmail, you should simply change the "From" part of the e-mail but I don't recommend doing this because these e-mails could be considered as spam since the server sending them is not a registered host for the e-mail addresses in the "From".
Trust me, this is not easy, even a real legit e-mail server must have registered reverse DNS entries so the e-mails it sends are not considered spam by other servers.