[Mimedefang] filter for header line

[MAYER Hans]

Dear mailing list members,

I am looking for a possibility to filter for an existing header line. 
I found a lot of examples where $Subject, $Sender and others can be used. 
But I need the possibility to find a certain header line, for example  X-Someinfo: 
Can someone point me to some examples. 
I was searching at Google but obviously with the wrong keywords. 

Kind regards 
Hans

--