Event invitations are not signed or encrypted with Microsoft Entourage
Microsoft Entourage 2004 includes the ability to sign an outgoing mail message with a digital signature, so that the recipient can verify that you sent it. It can also encrypt messages, so that other people cannot read them in transit.
Entourage does not sign or encrypt event invitations you send -- even though invitations are a special form of message sent to others. It does this to prevent interoperability problems. If you compose an event invitation, you will not be able to select the normal security options that you would have when composing regular e-mail messages.