Adding new rule for emails containing header Is it possible to create a new rule for mail to perform an action if the message contains a custom header?  

I want to automaticaly detect emails that contain the "X-hashcash" header.
Yes.

If you go to Preferences Rules and Add Rule then the first of the condition pop up lists has Edit header list.. as the last choice. In the dialog click on the + and add in X-hashcash. Done.