Illegal Microsoft Exchange Server command : XEXCH50

[hotice_]

Hi,
Ever since we enabled Messaging Security, I've noticed that we have been unable to send emails outbound to specific domains.

This is the error message that I get in the logs:
Illegal Microsoft Exchange Server command : XEXCH50

It appears Messaging security is blocking these specific Exchange messages required for authentication. Apparently, disabling this is not a viable or recommended option although it is unclear and I can't find any firm documentation explaining this

Has anyone ever encountered this..and/or have a workaround?


Running NGX R65 w/ Messaging Security on SPLAT 2.4

Thanks