[BUGFIX] Sending fails on multiple email-addresses

Specifying multiple recipients (To or CC) for emails
leads to an error upon form-submit without further explanation.
This patch implements validation for multiple email addresses of
which only the valid ones are then returned.
The sending-API can already handle multiple recipients.

Change-Id: I59b65adc4e09d298167738778c17d310b8ecce47
Fixes: #44485
Releases: 4.7, 6.0, 6.1
Reviewed-on: https://review.typo3.org/17505
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Tested-by: Stefan Neufeind
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert