Issue with calling MTOM services hosted on Axis2 1.6.2 from an Axis2 1.5.6 client

Has anyone encountered an issue trying to call an Axis2 1.6.2 service that returns an MTOM attachment from an Axis2 1.5.6 client?  We are trying to upgrade our services to use the latest version of Axis2 and this is causing a big issue that might prevent us from performing the upgrade.  Basically, what we are seeing is that everything looks good on the server (1.6.2), but on the 1.5.6 client it is failing the security check on the response.  The Fault we are getting is this:

org.apache.axis2.AxisFault: The signature or decryption was invalid

Thanks,
Wally