Spamming By “Email This Article” Feature

In Simple Thoughts blog and many other blogs worldwide there is a feature to email an article you liked (or hated) to your friends. Recently I noticed that this feature is being abused to send SPAM. How does it work?
When you email any article to anyone, you are offered a choice to write your own comments accompanying the article. This is being used by spammers to send their spam missive. The damaging part is that your mail server is being used to commit their nasty deed. If anyone suffers from this it would be you, the innocent blogger and the recipients of the spams.
A simple solution to this would be to use CAPTCHA. However for now I have removed the code which posts your accompanying message. So at least no spam can be sent. The email will still go with the article details. I need to think more about a better solution to this problem. Any ideas are welcome.