The "targetAddress" attribute is being populated with improper information for synchronized stub objects, the value is not the value of the primary SMTP address, it is different.
QCS assigns (populates) the targetAddress attribute not by using the primary SMTP address value, it populates it based on the settings you have specified under Namespaces.
This is the only way in QCS to specify which namespace should be used when populating the targetAddress, QCS will use the first available address which has been specified under Namespaces and has been found on synchronized objects.
For detailed instructions please review the User Guide, page 57 for version 3.0 and pages 73-74 for earlier versions, the part Namespaces, it contains a screenshot and a good description.
Excerpt from the User Guide:
For each original object, Collaboration Services creates a stub object in the forests that subscribe to the collection. The mail received by this stub should be redirected to the original object's e-mail address.
Since mailbox-enabled object can have more than one proxy address, Collaboration Services should select which of them is to be used for mail redirection from stub object. To choose the redirection address, Collaboration Services retrieves the particular object's primary or secondary SMTP addresses while publishing an object and checks which of the specified Namespaces (starting from topmost namespace in the list) matches the retrieved object's SMTP addresses. The first matching proxy address will be selected as the redirection address for the current object.
If none of the retrieved addresses is encountered in the specified namespace, a stub object will not be created.
Note: if it is desired to populate the targetAddress attribute with the value of the primary SMTP address then the primary SMTP address should be placed on top under Namespaces.
Please also review the following solution on this topic:
Solution SOL29083 - Which proxy addresses will be used for mail redirection on the stub objects, when multiple proxy addresses exist on the original objects? - https://support.quest.com/SUPPORT/index?page=solution&id=SOL29083