[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.

This change is for 1.9.x branch.

Change-Id: Ic731e92cfed32fdf0a5ccd708266ae9dff8572f2
Resolves: #56047
Releases: 1.9.6, 1.10.0
Reviewed-on: http://review.typo3.org/28747
Reviewed-by: Simon Schaufelberger <simonschaufi@googlemail.com>
Tested-by: Simon Schaufelberger <simonschaufi@googlemail.com>