If 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 is present.

RFC2821 specifically recommends against the use of the Apparently-To: header, so that header should never be defined in the configuration file.