I think with most services, the from email address will be the service's address, not the one filling it out. If it allowed changing the from address to anyone else's entered, those emails would most likely be filtered as spam by most servers since the email address would be considered "spoofed." Then again, my form's auto-reply says it's from adam@coffeecuphelpDOTnet, so who knows.

If yours is blank, there might be a PHP setting on the host server that needs tweaking.