13. The apparatus as recited in claim 8 , wherein the origin information comprises any of a network address of a sender of the electronic mail message, a hash value of the network address of the sender of the electronic mail message, or a message header “From:” value that is extracted from the electronic mail message.
8. A data processing apparatus, comprising: one or more processors; an HTTP proxy server hosted on one or more of the one or more processors; an SMTP mail transfer agent that is coupled to the HTTP proxy server; electronic mail processing logic that comprises one or more sequences of instructions which, when executed by the one or more processors, cause the one or more processors to perform the steps of: receiving an electronic mail message that comprises one or more hyperlinks; modifying each of the one or more hyperlinks in the electronic mail message by associating, with the one or more hyperlinks, an identifier value that indicates origin information; delivering the modified electronic mail message to a recipient account; in response to receiving a user request to access a specified hyperlink in the modified electronic mail message, retrieving the associated identifier value for the specified hyperlink, and determining whether access to the specified hyperlink is allowed based on the retrieved associated identifier value.