A system for making the contents of the “Subject:” field of a response email message more meaningful and more useful within a conversation thread of email messages by providing an abstract of the response message within the “Subject:” field. Before a response message is sent, the system automatically generates at least one new subject line that it proposes to the user for use in the “Subject:” field of the response message. The system generates a user interface display of a number of proposed subject lines, and also provides the user with the options of accepting, modifying, or declining the proposed subjects lines. The displayed contents of a “Subject:” field for the response message contains the currently selected one of the proposed subject lines. A predetermined one of the proposed subject lines may be provided as an initial default value of the “Subject:” field. The system generates the proposed subject lines using auto-summarization techniques, which are performed using various text analysis and information retrieval techniques based on attributes of the response message.