A collaboration system is described for sharing files. The collaboration system operates by receiving an Email message from a sender, the Email message specifying an attachment item (such as a file) and one or more recipients. The Email message also specifies a sender code assigned to the sender for the particular attachment item. The collaboration system stores the attachment item and then generates a recipient-specific code associated with each recipient who is to receive the Email message. The collaboration system then sends a recipient-tailored Email message to each recipient. Each recipient-tailored Email message contains Email content provided by the original Email message along with a recipient-specific code associated with the recipient. Each recipient can access the attachment item via its recipient-specific code, which constitutes a secure access key.