When processing inbound, DKIM signed messages, if the message has an empty body the message fails DKIM vlidation. This only affects messages with a zero length body, not messages with normal email body content or messages with only whitespace characters in the body.
Messaging Gateway
Messaging Gateway does not use the "relaxed" DKIM algorithm when processing inbound messages with a zero length body even though the message has been signed using the "relaxed" algorithm.
This issue has been addressed with the SMG 10.6.4 release.