Name

Subject:

Synopsis

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

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