Email changes in textbox to html code

Hi there I got a little bug in my contactform. Everytime I want to send my form an get an error cause the security code is wrong my email in the email textbox changes from mail@example.com to

<script type='text/javascript'> <!-- var prefix = 'ma' + 'il' + 'to'; var path = 'hr' + 'ef' + '='; var addy3008 = 'mail' + '@'; addy3008 = addy3008 + 'example' + '.' + 'com'; document.write('<a ' + path + '\'' + prefix + ':' + addy3008 + '\'>'+ addy3008+ '<\/a>'); //-->\n </script><script type='text/javascript'> <!-- document.write('<span style=\'display: none;\'>'+ 'Diese E-Mail-Adresse ist vor Spambots geschützt! Zur Anzeige muss JavaScript eingeschaltet sein!'+ '</'+ 'span>'); //--> </script>

What is that? And what did I do wrong?

GreyHead you`re the best... It seems that this had been the error.
Many thanks