novice with hacked email form issue

An email form that uses a simple server side php code to send the variable values managed to send: Content-Type: multipart/alternative; > boundary=5c7c7e682d991e8ec1f6825582ea2103 > MIME-Version: 1.0 > Subject: round a rock by way of anchorage > bcc: charieses329@aol.com > > This is a multi-part message in MIME format. > > --5c7c7e682d991e8ec1f6825582ea2103 > Content-Type: text/plain; charset=\"us-ascii\" > MIME-Version: 1.0 > Content-Transfer-Encoding: 7bit > > system expict th time is > --5c7c7e682d991e8ec1f6825582ea2103-- > This appears between responses to "$msg.=" and shouldn't be something the user could see. In order to figure out how to prevent this, I need to know how it was done. I know I need to validate the email address more closely, but I'm curious as to what created this. I've found some interesting articles on the web, but nothing seems to deal with this issue. Pointing me in the right direction would be appreciated! TIA!