[BUGFIX] PM notification mail broken

In version 1.9.5 sending a PM results in the TYPO3 error "Address
in mailbox given [] does not comply with RFC 2822, 3.6.2."

This seems to be due to the change of the mailengine.

Related: #56047
Releases: 1.9, 1.10
Change-Id: I639552d73ff416e89ebf5f77c3e610e439726657
Reviewed-on: http://review.typo3.org/33343
Reviewed-by: Ines Willenbrock <icy@icys-scholle.de>
Reviewed-by: Simon Schaufelberger <simonschaufi@googlemail.com>
Tested-by: Simon Schaufelberger <simonschaufi@googlemail.com>