This extension extends each powermail form with a checkbox. This checkbox is placed before each submit button and linked to your privacy policy. The user has to accept your privacy policy to submit a form. The validation is also done by the server. The value is stored in the database. You can disable this checkbox for each form individually.

Feel free to test and give feedback.

Set your privacy policy page with a TypoScript constant.

plugin.tx_powermail.settings.privacyPid =

You can override the language labels via TypoScript

plugin.tx_powermail._LOCAL_LANG.en.tx_cspowermailgdpr.checkbox.label = I accept the ###privacy policy###.

or in an extra locallang file

$GLOBALS['TYPO3_CONF_VARS']['SYS']['locallangXMLOverride']['EXT:powermail/Resources/Private/Language/locallang.xlf'][10] = 
    'EXT:myext/Resources/Private/Language/locallang.xlf';

There you can override tx_cspowermailgdpr.checkbox.label. ### will be replaces by the link, e.g.:

I accept the ###privacy policy###.

becomes

I accept the <a href="privacypolicy" target="_blank" title="Show privacy policy">privacy policy</a>.