Helps to read about SMTP.
Messages are placed in envelopes when transferred from server to server. Thus if there is a problem handling the message (failure notices, bounces etc) it is reported to the "return address" called the envelope-sender. This means the envelope-sender doesn't necessarily appear anywhere in the headers of the message when it was first submitted at all.
The envelope sender and recipient are specified separately from the message headers. With SMTP, the server is told what the sender is using the "MAIL FROM" command. And when submitted using the sendmail interface the sender is specified using the -f parameter.
And the property of PHPMailer is aptly called $Sender.
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