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.
This would be displayed by the mailx(1) program
in truncated form as:
14 email@example.com 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