[TASK] Additional check for valid domain in validEmail()

If using a correct email address (= correct syntax),
but the domain name has no valid MX entry,
then a given email is validated as valid,
but the destination is still wrong. In some cases
the email should be completely validated.
By adding an additional PHP call to check if
the MX entry of the domain part is valid, the
validEmail() can be put in a stricter mode.

Releases: master
Resolves: #56771
Change-Id: I1396320835a9e2b51533d8b43115a7c4ef3ec338
Reviewed-on: http://review.typo3.org/28264
Reviewed-by: Frank Nägler <typo3@naegler.net>
Tested-by: Frank Nägler <typo3@naegler.net>
Reviewed-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Tested-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Reviewed-by: Benjamin Mack <benni@typo3.org>
Tested-by: Benjamin Mack <benni@typo3.org>