[Typo3] Mailformplus - defined emailHeader is not attached to header

[W van Bruggen]

Hello list,

I've used the mailformplus extension on a 3.7 installation which works
great, and i could use an html header for the emails. I'm am now
setting up a mailformplus form on a 3.6.1 installation and use the
exact same TS that i used on the 3.7 installation. But for some reason
the header does not get added to the emails. Is this a 3.6.1 specific
problem or is the problem somewhere else ?

This is the TS i use :
plugin.tx_thmailformplus_pi1.emailHeader = Content-Type: text/html;

greets,
Wietse van Bruggen