TYPO3 Core

1. How to reproduce

1. Setup a TYPO3 form with an email field
1. Add email validation
1. Go to the form
1. Enter "${@print(md5(security_test))}" as email address
1. Send the form
1. Form was sent successfully

As it's not a valid email address the email sender (website admin) gets a bounce mail.
This can be done very often to flood the email box.

1. How to solve

Add the DNSCheckValidation to the EmailValidator of library (egulias/EmailValidator).