The rollup provides the following fixes and improvements (https://support.microsoft.com/en-us/help/4091243/update-rollup-21-for-exchange-server-2010-service-pack-3)
This rollup is 2 security updates that resolve the below vulnerability
- This security update resolves a vulnerability in Microsoft Exchange Outlook Web Access (OWA). The vulnerability could allow elevation of privilege or spoofing in Microsoft Exchange Server if an attacker sends an email message that has a specially crafted attachment to a vulnerable server that is running Exchange Server
You can download this rollup from http://www.microsoft.com/downloads/details.aspx?FamilyID=f2c20ab1-3c99-4224-829f-cd8d8ae55031
