I used the e-mail message popup to send a message and I think I got this in return:

From Mailer-Daemon Thu Apr 27 17:05 PDT 1995
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Content-Type: text
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From: Mailer-Daemon (Mail Delivery Subsystem)
To: Postmaster
Subject: Returned mail: User unknown
Date: Thu, 27 Apr 1995 17:05:29 -0700

   ----- Transcript of session follows -----
<<< HELO Eng.Sun.COM
<<< MAIL From:<###@###.###>
<<< RCPT To:<Null Object@prosper>
550 <Null Object@prosper>... User unknown
<<< DATA

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Date: Thu, 27 Apr 1995 20:04:41 -0400
From: ###@###.### (Mail Delivery Subsystem)
Subject: Returned mail: Bad usage
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