The Smail 3.x program (a UUCP-oriented replacement for sendmail) produces an Apparently-From: header when it is unable to find any of the official sender headers in a mail message. The address that it provides to this nonstandard header is taken from the envelope of the message.

The sendmail program, on the other hand, places the envelope sender into a From: header in this situation. If there is no envelope sender and if the sender was not specified in the command line, sendmail sets the sender to be the postmaster.

The Apparently-From: header is mentioned here only because it can appear in messages received at sites that run sendmail. It shouldn't cause problems because a good sender address still appears in the SMTP envelope.

The Apparently-From: header should never be declared in the configuration file and should not be added to conf.c.