When migrating messages from GroupWise to Exchange, the Sent Date and Received Date are preserved but the modified date is not maintained.
The Modified date is changed to the migration date by Exchange when the messages are migrated. GME has no control over this setting.
Use one of the other date settings in your mail archive policy.
Refer to Microsoft KB Article ID: 295657 - "Outlook does not archive messages as expected based on received date"