[FEATURE] #5111 Check for empty or invalid e-mail addresses when sending

Change-Id: Ie0cb17e461d1121b2a5a69c03d0eee890b8c0b2a
Reviewed-on: http://review.typo3.org/32550
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>