How can we combat false x-envelope spam email

Hello all,
To explain the issue in detail&
We have had a recent influx of spam emails. When they arrive the recipient details are often an obscure, non-existent user with the correct @... suffix. The email, however, actually arrives at a real users inbox  still with this obscure user listed as the recipient in Outlook. When inspected, the message header uses a bit of trickery in the x-envelope. Whereas only one recipient appears to the user when they open the email, there is actually a string of recipients in the x-envelope. The effect the Spammers achieve is that many users in our company receive the same email  which on screen looks like it was meant to be delivered to somebody else.

Are you aware of any way in which we can tweak the rules on our email filter so that we can probe for such trick x-envelopes and stop them arriving with our users?
Any coments/thoughts much appreciated
Danny