This policy setting allows you to specify whether the Inbox application requires that all messages must be signed. This policy ...

"This policy setting allows you to specify whether the Inbox application requires that all messages must be signed. This policy is applicable only if you are using Microsoft Exchange 2003 SP2 or Microsoft Exchange 2007 SP1. The user must have a certificate on the mobile device which can be used to digitally sign outgoing e-mail messages.  If you enable this policy setting, then all messages must be signed.  If you disable or do not configure this policy setting, then messages do not have to be signed."