Anonymous Contact Form allows people that are not logged into the site to contact users via e-mail via a contact form.
ACF is a very bare bones module. There are no settings, one permission and one url format. Once installed, enable access permissions to the form for the anonymous user, and they can then use either of these paths to access it:
If the [uid] is not a valid user, a 404 Not Found error is returned.
Linking to these pages is entirely left to the site developer.