Active Directory object requires email attribute to be filled, Document protection with AD-RMS
While sending documents that are RMS protected, you need to make sure that the email attribute of the person sending the document in Active Directory is filled.RMS is reliant on IIS and is a web based technology the client requires an email address attribute in Active Directory as this is what RMS uses to identify users.
The link: https://support.risualblogs.com/blog/2010/02/03/ad-rms/ describes RMS technology, how does it work, recommendations to best benefit from its features, and the integration with Active Directory and SQL server