zmailer bounces being sent To: @

[Arnt Gulbrandsen <arnt@trolltech.com>]

One of my Zmailer hosts is generating those, and I can't figure out where
that "@" comes from.  Zmailer 2.99.54p1.

Here's a typical example, indented for clarity.

    rcvdfrom STDIN ("??"@lupinella.troll.no)
    channel error
    todsn ORCPT=rfc822;@
    to <@>
    env-end
    To: @
    From: The Post Office <postmaster>
    Sender:	mailer-daemon
    Subject: Delivery reports about your email [FAILED(1)]
    MIME-Version: 1.0
    Content-Type: multipart/report; report-type=delivery-status;
    	boundary="S114382AbRBIJJB=_/lupinella.troll.no"
    
    This is MULTIPART/REPORT structured message as defined at RFC 1894.
    
    Ask your email client software vendor, when will they support this
    report format by showing its formal part in your preferred language.
    
    --S114382AbRBIJJB=_/lupinella.troll.no
    Content-Type: text/plain
    
    This is a collection of reports about email delivery
    process concerning a message you originated.
    
    Some explanations/translations for these reports
    can be found at:
          http://www.zmailer.org/reports.html
    
    FAILED:
      Original Recipient:
        rfc822;niagra4899@hotmail.com
      Control data:
        smtp hotmail.com niagra4899@hotmail.com 65534
      Diagnostic texts:
    ...\
        <<- MAIL From:<@> SIZE=10582
        ->> 250 Requested mail action okay, completed
        <<- RCPT To:<niagra4899@hotmail.com>
        ->> 550 Requested action not taken: mailbox unavailable
    
    Following is a copy of MESSAGE/DELIVERY-STATUS format section below.
    It is copied here in case your email client is unable to show it to you.
...

Naturally, these files immediately cause another fault.

Any ideas?

--Arnt