1. A method for communicating e-mail messages using a secure e-mail service, comprising: receiving an e-mail message from a sender addressed to a recipient; receiving a request for access to the e-mail message from an e-mail client associated with the recipient, wherein the e-mail client is executed by a recipient computing device located remotely from a server system that implements the secure-e-mail service and wherein the e-mail client is executed by a sender computing device, the request including a message access key, wherein the message access key contains non-secret information which describes the message, the secure e-mail service from which it is to be retrieved, and a unique message ID that identifies it on the service; in response to receiving the request for access to the e-mail message: sending to the e-mail client a request for password input; receiving a password input from the e-mail client; and validating the password input; upon validating the password input, providing the e-mail client with access to the e-mail message as identified by the message access key, where providing the e-mail client with access to the e-mail message includes sending the e-mail message to the e-mail client via a secure HTTPS protocol with instructions for temporary local storage; receiving a navigation notification indicating a navigation away from a display pane of the e-mail client in which the e-mail message is displayed, the navigation being directed to a different portion of a user interface in which different content is displayed, followed by a navigation returning to the e-mail message; and in response to receiving the navigation notification: withholding access to the e-mail message by the e-mail client, and providing the e-mail client with a message placeholder, where withholding access to the e-mail message by the e-mail client includes sending a command to delete the e-mail message from the recipient computing device; after providing the e-mail client with the message placeholder: receiving a second request from the e-mail client to access the e-mail message a second time; determining if a permission of the e-mail message allows a second access to the e-mail message by the e-mail client; when the permission allows the second access to the e-mail message, providing the e-mail client with access to the e-mail message for the second time; and when the permission does not allow the second access to the e-mail message, continuing to withhold access to the e-mail message by the e-mail client.