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:

• The message must have an attachment
• In the MIME headers for the attachment the Content-Type header must not have a "name" attribute
• The message, including attachments, must be over 128KB in size

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.