Allow mails from non-registered users

I have a classified ads site with an link-option to mail the seller directly, placed in each individual ad. When a visitor clicks that option a log in screen comes up, so only registered users will be able to send a mail directly to the seller/advertiser. But I would like this to be altered in a way that every visitor (registered and non-registered) can send a mail to the seller/advertisor when having a question about the object offered in the ad. I have searched my admin and the internet but see no solution for this. Thanks in advance for your help !