We are not sure what you mean by the “x-classification custom header”, could you please provide more information. Do you set the classification using the message “Send Options”?
Currently in GroupWise, we don’t think its possible to add classification to the message header. However, Novell has added the x-field support in Bonsai (GroupWise 8 onwards) which can be use to add the classification. The value of a x-field is just the IMAP header string.
Just a thought, when you send the classification messages in GroupWise, you can change the mime encoding to ISO. See the PrefAdvanced token for more information.