Could visitors / guest send a message to a member?

In our social network we need that visitors without register can send messages to an registered member.

Obviously to avoid spam, captcha function must be included.

We are not talking about post comments or comment a photo. I mean, our concern is let visitors could send private messages to members.

We have not found any plugin to do this... or maybe we have not searched with right keywords ;-) Anyway... anybody can help us?

Thnx in advance!

jordi