The attached patch adds DKIM checks to the backscatter detection in the VBounce plugin, in addition to 
using the existing content heuristics.

The idea is that if a site uses DKIM to sign outgoing messages, these signatures should be contained in 
incoming bounces (if the bounces contain a copy of the original message, or at least its headers). 
Additionaly, one can specify additional headers to check for - if a bounce message does not contain the 
header, it might be backscatter (e.g. "X-MyAntiBackScatterHeader").