[fetchmail] 553 errors when delivering certain mail

Hi,

I am using fetchmail to consolidate many email accounts into a single 
inbox. I noticed that some messages are not properly deliverd. The 
messages that 'disappear' have defective Return-Path headers. Here is an 
example header of a message that fetchmail chokes on (it's an error 
message generated by sendmail on a server I administer):

  Return-Path: <@mail.knet.ca>
  X-Sieve: cmu-sieve 2.0
  Received: from localhost (localhost)
       by mail.knet.ca (8.11.6/8.11.2) id h82E4u607162;
       Tue, 2 Sep 2003 10:17:24 -0500
  Date: Tue, 2 Sep 2003 10:17:24 -0500
  From: Mail Delivery Subsystem <MAILER-DAEMON(_at_)mail(_dot_)knet(_dot_)ca>
  Message-Id: <200309021517(_dot_)h82E4u607162(_at_)mail(_dot_)knet(_dot_)ca>
  To: postmaster(_at_)mail(_dot_)knet(_dot_)ca
  MIME-Version: 1.0
  Content-Type: multipart/report; report-type=delivery-status;
       boundary="h82E4u607162.1062515844/mail.knet.ca"
  Subject: Postmaster notify: see transcript for details
  Auto-Submitted: auto-generated (postmaster-notification)

Here is the verbose log of what happens when fetchmail processes this type 
of message:

  fetchmail: IMAP> A0024 STORE 1 +FLAGS (\Seen \Deleted)
  fetchmail: IMAP< * 1 FETCH (FLAGS (\Deleted \Seen))
  fetchmail: IMAP< A0024 OK Completed
  fetchmail: IMAP> A0025 EXPUNGE
  fetchmail: IMAP< * 1 EXPUNGE
  fetchmail: IMAP< * 4 EXISTS
  fetchmail: IMAP< * 0 RECENT
  fetchmail: IMAP< A0025 OK Completed
  fetchmail: IMAP> A0026 FETCH 1 RFC822.HEADER
  fetchmail: IMAP< * 1 FETCH (RFC822.HEADER {604}
  reading message 8 of 11 (604 header octets)
  fetchmail: SMTP> MAIL FROM:<@mail.knet.ca> SIZE=108757
  fetchmail: SMTP< 553 5.1.3 <@mail.knet.ca>... User address required
  fetchmail: SMTP error: 553 5.1.3 <@mail.knet.ca>... User address required
  fetchmail: SMTP< 220 yoda.adis.on.ca ESMTP Sendmail 8.11.6/8.11.6; Tue, 2 Sep 
2003 16:47:39 -0500
  fetchmail: SMTP> HELO localhost
  fetchmail: SMTP< 250 yoda.adis.on.ca Hello localhost [127.0.0.1], pleased to 
meet you
  fetchmail: SMTP> MAIL 
FROM:<FETCHMAIL-DAEMON(_at_)yoda(_dot_)adis(_dot_)on(_dot_)ca>
  fetchmail: SMTP< 250 2.1.0 
<FETCHMAIL-DAEMON(_at_)yoda(_dot_)adis(_dot_)on(_dot_)ca>... Sender ok
  fetchmail: SMTP> RCPT TO:<@mail.knet.ca>
  fetchmail: SMTP< 553 5.1.3 <@mail.knet.ca>... User address required
  fetchmail: SMTP> RSET
  fetchmail: SMTP< 250 2.0.0 Reset state

  fetchmail: IMAP< )
  fetchmail: IMAP< A0026 OK Completed

> From reading the fetchmail man page, I understand that fetchmail examines 
the mail headers to deterime a proper from address for the envelope. How 
properly retrieve and deliver these messages? Note that this is a busy 
mailbox and these sendmail generated emails make up only a small part of 
the overall emails.

Thanks,
Adi