Strange problem with mail sent from Exchange 2003 server

Hi,

I'm suffering a rare problem when I receive e-mails from a exchange  
2003 server to my qmail-ldap server.

Many mails sent from this server have this problem:
-----------------------------------------------------------------------
220 mx.mydomain ESMTP
EHLO exchange2003
250-mx.mydomain
250-PIPELINING
250-SIZE 26214400
250-DATAZ
250-STARTTLS
250 8BITMIME

MAIL FROM:<xxxxxxxx@exchange2003> SIZE=1433152
250 ok
RCPT TO:<me@mydomain>
250 ok
DATA
354 go ahead punk, make my day

// (1200seconds later...)

451 timeout (#4.4.2)


The exchange 2003 server can send without problem to other servers -as  
gmail.com or pobladores.com-.

It isn't a problem of mail size restriction or antispam policy. The  
delivery of a mail can fail X times and finally send ok. Others,  
expires after two days retrying.

Have you any idea?

Regards,
Spamis