Forum Receiving blank emails
 

For a week or so i have been getting blank emails sent to me from my forum.
I noticed several ips located at "Send Forum Feedback" or sendmessage.php
I did have non registered able to use the contact form.
I have since turned that off.

Now i banned a heap of ips and it seems to have slowed down.
Now i am getting an email every 20 minutes on the dot.
For the life of me i cant work out why or how.

Any ideas?