TYPO3 Core

The purpose of GeneralUtility::validEmail() as stated in the comment is: "Checking syntax of input email address".
#56771 introduced a mechanism to validate if the underlying infrastructure (DNS) can verify if an MX record was set for the domain part of the given email.

This feature should be reverted for several reasons:
1. The MX query check is beyond the scope of the function.
2. Validating email addresses by coupling the underlying DNS infrastructure is brittle and subject to fail on certain environments (no network, no proper DNS resolver, intranet, local dev, ...). It might lead to false negatives, which is for example the case for root@localhost
3. The corresponding unit tests are also brittle, since they rely on a proper DNS setup for the test fixtures and available DNS resolver in test environment.