the header of a mail message lacks recipient information (lacks all
of the To:, Cc:, and
Bcc: header lines), sendmail
adds an Apparently-To: header line and puts the
recipient's address from the envelope into the field
of that line. This behavior is hardcoded into pre-V8.7
sendmail, but beginning with V8.7, it can be
tuned with the NoRecipientAction option (NoRecipientAction).
The Apparently-To: header name is not defined in
RFC2822. It is added by pre-V8.7 sendmail
because RFC2822 requires at least one
To: or Cc: header, and neither
RFC2821 specifically recommends against the use of the
Apparently-To: header, so that header should
never be defined in the configuration file.