Custom CDO properties persisting outside of organisation

[BillyBushey]

We track emails internally by allocating them a file reference number as a custom property. This works fine using MAPI in Outlook.
Some emails are automated from our server, and so are sent using CDO. This can't set custom properties, although it can add custom X-Headers instead, and then I can use an event sink in Exhange to set the custom property.
Once this email is sent to an external recipient, I'd like to track any replies they send, ideally by persisting this custom property.
I don't want to use ConversationTopic - this is not as reliable as a unique reference number.
Is there any way I can set a custom property that will persist outside the organisation and still be there when we receive a reply?

We use SBS 2003 in our organisation.

thanks
Bill