Create a message with restricted permission
To restrict permission to content in a message, you must have Microsoft Office for Mac Standard 2011.
Information Rights Management (IRM) allows authors to restrict permission for e-mail messages, documents, workbooks, and presentations on a per-person or per-group basis. In Outlook for Mac 2011, you can restrict a message from being forwarded. Depending on how IRM is used in your organization, you may be able to set other restrictions, such as not allowing recipients to Reply All to a message.
On the Home tab, click New, and then click E-mail Message.
On the Options tab, click the arrow next to Permissions, and then click the type of restriction that you want.