If you delete a contact from your Guest List after an Invitation has been sent, the email will still remain in the guest's inbox, but he/she will no longer have access to the Greenvelope invitation. If the guest tries to open it, he/she will be taken to a page that says, "We're sorry, this Greenvelope is no longer available."

Contacts deleted from the Guest List will remain in your Address Book (found in the top right of the Guest List page) and can be easily restored to the event's Guest List.