Can't create output: Error 0 (Procmail is Local Delivery Agent)

Hi. I run Procmail as Local Delivery Agent at the Mail Hub at work.

Now and then, not often, I get the following error message to Postmaster
for high-volume users --- Postmaster itself and a user `drift':

***************************************************************************
   ----- The following addresses have delivery notifications -----
drift(_at_)u30003(_dot_)rsv(_dot_)svskt(_dot_)se  (unrecoverable error)
    (expanded from: root)

   ----- Transcript of session follows -----
procmail: Error while writing to "/usr/mail/drift"
 From uucp  Tue Apr  2 23:45:04 1996
  Subject: uucleanup ran; /usr/spool/uucp du
   Folder: **Bounced**                                                      823
 550 drift(_at_)u30003(_dot_)rsv(_dot_)svskt(_dot_)se(_dot_)(_dot_)(_dot_) 
Can't create output: Error 0

  [ Part 2: "Included Message" ]

Reporting-MTA: dns; u30003.rsv.svskt.se
Arrival-Date: Tue, 2 Apr 1996 23:45:04 +0200 (METDST)

Final-Recipient: RFC822; drift(_at_)u30003(_dot_)rsv(_dot_)svskt(_dot_)se
X-Actual-Recipient: RFC822; drift(_at_)u30003(_dot_)rsv(_dot_)svskt(_dot_)se
Action: failed
Status: 5.3.0
Last-Attempt-Date: Tue, 2 Apr 1996 23:45:06 +0200 (METDST)
***************************************************************************

Another:

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
   ----- The following addresses have delivery notifications -----
\postmaster  (unrecoverable error)
    (expanded from: postmaster)

   ----- Transcript of session follows -----
procmail: Error while writing to "/usr/mail/postmaster"
 From daemon  Wed Apr  3 23:45:24 1996
  Subject: Returned mail: User unknown
   Folder: **Bounced**                                                     7023
 550 \postmaster... Can't create output: Error 0
 550 MAILER-DAEMON... User unknown

  [ Part 2: "Included Message" ]

Reporting-MTA: dns; u30003.rsv.svskt.se
Arrival-Date: Wed, 3 Apr 1996 23:45:23 +0200 (METDST)

Final-Recipient: RFC822; postmaster(_at_)u30003(_dot_)rsv(_dot_)svskt(_dot_)se
X-Actual-Recipient: RFC822; 
postmaster(_at_)u30003(_dot_)rsv(_dot_)svskt(_dot_)se
Action: failed
Status: 5.3.0
Last-Attempt-Date: Wed, 3 Apr 1996 23:45:25 +0200 (METDST)
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


Any clues?



-- Leif Erlingsson, http://www.geocities.com/RodeoDrive/1998, +46 8 604 0995
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