1. A method for automatic generation of subject lines for electronic mail (email), the method comprising: extracting topics from an email message; computing a sender relevance score for each topic; ranking the topics based on the sender relevance scores; ranking a plurality of syntactic units from the email message based on the topic ranking; assigning one or more subject lines to the email message based on the ranking of the syntactic units; assigning different subject lines to the email message for each respective intended recipient of a plurality of intended recipients of the email message so that each respective intended recipient is sent the same email message with a different subject line from that of other intended recipients; wherein the different subject line corresponding to an intended recipient is determined based on one or more characteristics of the intended recipient; and assigning an identifying subject line different from the one or more of the subject lines assigned to the email message, the identifying subject line indicating one or more preferences of a sender of the email message and being visible to the sender and not visible to a recipient of the email message; wherein, when the sender receives a reply to the email message from the recipient, the reply email message includes the identifying subject line instead of the one or more of the subject lines assigned to the email message, or a combination of the identifying subject line and the one or more of the subject lines assigned to the email message.