Target Exchange organization has RUS policy configured with secondary smtp address but it is not getting stamped on migrated objects. However this secondary smtp address is getting stamped successfully on newly created mailboxes.
This behaviour is by design of Microsoft Exchange server. Please refer to the resolution section for more details.
The behaviour is described in the following Microsoft article:
"This behavior occurs because the Recipient Update Service determines that the migrated user already has the proxyAddresses attribute populated. Because the attribute is populated, secondary SMTP addresses are not generated. Secondary proxies are added only when one of the following conditions is true:
-The recipient has nothing at all in the proxyAddresses attribute. For example, the recipient is brand new in the organization or the recipient wasn't migrated.
-The secondary proxies are present in the gatewayProxy attribute on the domainNC Recipient Update Service object. This means that the recipient policy is being applied against the organization.
To work around this behavior, manually apply the recipient policy. To do so, follow these steps:
1. Click Start, point to Programs, point to Microsoft Exchange, and then click System Manager.
2. Expand Recipients, right-click the policy that you want to apply, and then click Apply this policy now."
Please also refer to this article for more details on how RUS works in Exchange 2000/2003: