The Subject: header can be included in mail messages to give the topic of the message. Most user mail-reading programs display the arbitrary text that forms the field of this header when listing received messages. Although such text can legally extend over multiple indented lines, most mail-reading programs recognize only the first such line:

Subject: About yesterday's meeting, I had some second
       thoughts about why the shape of the bonnet should
       remain so sharply curved at the ends.
    
  whitespace

This would be displayed by the mailx(1) program in truncated form as:

14   gw@wash.dc.gov Fri Aug  7 12:57  22/770 "About yesterday's meeting"

The Subject: header is not used by sendmail, but it is often wrongly (albeit harmlessly) included in the configuration file:

HSubject:         this actually does nothing