Depending on your exchange server's settings, it may allow you to input any email address in the from field.
However, in your situation, you'll need to send the email manually or if your using on-line forms you can probably use a macro in the From email for the email notification referencing the email entered by the user.
You can check if you can send emails using any email in the from address by going to System > Emails and sending out an email using any email address. Keep in mind there are restrictions on your email server such as if you don't use an email from the exchange server, it may fail.