[Typo3] direct mail: mailformat disappears, no newsletter is sent

[Matthias Mark]

The solution

There is the following thread in typo3.org / german userlist:

Mail versenden in TYPO3 3.7.0 und safe mode=off
the link is:
http://typo3.org/documentation/mailing-lists/german-user-list-archive/thread/110109808/?tx_maillisttofaq_pi1%5Bsword%5D=Mail%20versenden%20in%20TYPO3%203.7.0%20und%20safe%20mode%3Doff&tx_maillisttofaq_pi1%5Banswered_only%5D=0&tx_maillisttofaq_pi1%5Bmode%5D=1

I had used this thread already because my formmail didn't work. 
After comparing the new Typo3 installation with an older one I noticed that 
I also changed the t3lib_htmlmail in the folder
typo3/t3lib and not only in t3lib. 
With using only the change in t3lib formmail works correct.

So I tried it with changing the same class in typo3/t3lib and 
now newsletters  (text and HTML) are sent.