Adding a disclaimer for all outgoing emails in Exchange 2013 Standard Edition

I tried to add a disclaimer in "mail flow" but it gave me the following error. Do I ready need an Enterprise edition to do this since adding a disclaimer seems a simple task for an email server? If I cannot add a disclaimer via mail flow, is there other alternatives to achieve it. We only have the Exchange Standard edition. Thanks

Error Message:
Rights Management Services (RMS) is a premium feature that requires an Enterprise Client Access License (CAL) for each user mailbox