Take a look to this IBM Technote # 1586180 :
Blank return receipts sent to external sender
We disabled the internal Domino disclaimer management (a setting in the configuration document inside the names.nsf) and reloaded the Domino router config (tell router update config). Now it works.
BTW, we use the fabulous third-party product called Crossware Mail Signature to manage all signature needs: simple and effective.