Email Problems

1. Whenever I try to "Invite a Friend" to my site, I get an "Email Send Failed" error message. I can't seem to find a reason why.
   
   If you're already logged in as a member, would the fields "Your Name" and "Your Email" automatically be filled up? Mine doesn't do that. If I try to fill them up and then type in my friends email address in the "Friend Email" box and then I click on "send letter", I will then get the error message "Email Send Failed".
   
2. I get the same error message when I try to "Send Letter" (This is from Actions in member's profiles). However, the "Your Email" and "Your Name" fields are automatically filled up. The error message will appear when I try to send the message.