After you create the new mail contact, the Windows SBS Email Address Policy will stamp the contact with the external SMTP address as a secondary address:
When you send an email to this SMTP address, the Exchange Transport service will deliver to the primary SMTP address. In the example above, if I send to
, Exchange will deliver to mylist@companyweb. At that point the email is forwarded to the drop directory, picked up by Sharepoint, and the attachments are delivered to the “mylist” document library.
This has the potential to be broken if you manually edit the Windows SBS Email Address Policy, which applies your SMTP domain address to all recipients.