> The error is on the senders Exchange server. We have no
> record of an error in the postfix logs.

Nothing at all?
 
> The sender admin said that when he does a telnet 25 test
> message, it send successfully, but then the connection
> freeezes and eventually times out with the 421 error. I
> have not been able to reproduce it.

debug_peer_list  = ip.of.the.exchange
then enjoy the increased logging for all traffic from ip.of.the.exchange

-- 
_________________________________________________

  Charit� - Universit�tsmedizin Berlin
_________________________________________________

  Ralf Hildebrandt
   i.A. des IT-Zentrum | Netzwerkdienste
   Stabsstelle des Klinikumsvorstandes
   Campus Mitte
   Schumannstr. 20/21 | D-10177 Berlin
   Tel. +49 30 450 570155 | Fax +49 30 450 570916
   [EMAIL PROTECTED]
   http://www.charite.de


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