Message intialization in business process

I've created a business process in XI 3.0 (SP 15) which processes a XML file and sents it in Idoc format to R/3.

Now I want to set up an exception handler branch to get possible errors which might happen during the mapping (transformation process).

In the exception handler branch I put in:

1)transformation step to fill a mail XML structure (standard XI ximail30_xsd.xsd)

2)send step to send the mail

3)control step to terminate the process

In any error case XI doesn't perform the first transformation step in this branch because the necessary source message for the mapping is not initialized. Even if I fill the target message only with constants this process doesn't work.

Any idea to get the message initialized? Or a total different solution for error handling? We don't want to use alert steps!

Thanks,

Christina