Messages which match a very specific set of criteria will be signed with invalid DKIM signatures resulting in their rejection from external mail systems which do sender authentication via DKIM signature validation.
For a message to have its DKIM signature malformed, the message must meet three criteria:
This issue is addressed with SMG 10.0.3 release. There is no workaround other than to disable DKIM signing for all messages if you are with SMG 10.0.2 and the under.