send email from public to user plugin answering classifieds

I’m doing a classifieds style website.
Anybody wanting to list, must sign up first. But I want members of the general public, without having to signup, to be able to send email to communicate to a user on what they have listed. It’s almost like the ability of leaving comments without signing up but at least the user will know the sender’s email address in order to reply to them.

Any plugin out there like that? I’ve searched for one for some hours but to no avail.

TIA