In the Form Submission Settings section, if you attempt to set an "Email field" (like "email"), and save the settings, it throws an error indicating that it must be a valid email address. But in fact, this refers to "The name of the field in the contact form that contains the email address of the sender", so it very much should be a field like "email".
I was able to get it to work by setting form.* in the local configuration keys setting, and then setting this value manually in the config.toml file.
Just wanted to pass this along. The form otherwise is very handy!