[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.

Change-Id: I92123206472855a13bc9b47ab4ef5b17e3c95c71
Resolves: #56047
Releases: 1.9.6, 1.10.0
Reviewed-on: https://review.typo3.org/28703
Reviewed-by: Simon Schaufelberger
Tested-by: Simon Schaufelberger