Some email addresses that contain certain characters, such as the ampersand, may return as invalid recipients although the address is valid.
550 5.1.1 Recipient address rejected: User unknown
When querying the data source, rather than pass the address to the data source as an unmodified string of characters, the email address is being passed to the data source as encoded xml data i.e. an '&' is being replaced in the query with '&' resulting in a failure to match the existing recipient address record in the data source.
This issue has been addressed with the SMG v10.0.2 release.