You can email to a user at a site from search results or any other listing by clicking Email link. Then you'll be transferred to a mailbox to send the email. You can also attach the file or image to the email.
If you haven't been to a site when someone wrote a message to you, you'll get an alert on the message to your email. You'll also see a blinking sign at the top of the page. You'll be able to click it and access all your messages.