TYPO3 Core

I don't think so. I want to block email adresses of providers for temporary email adresses like trash-mail.com or milinator.com. Additionaly I want to check the MX Record of the domain from the email address.

And for this it would be nice to use t3lib_div:validEmail and not a selfcoded class and don't need to change in Extensions which use the t3lib_div:validEmail the used function to my own validate function.

Ok I see the point here, however t3lib_div::validEmail() really just checks for the validity of an email using PHP's filter_var() function.

I think what you want to do makes sense, but should not be in t3lib_div as it would just bloat up that class even more. Instead, how about going forward and creating an email validator class that can be extended?