Question : Can't delivery email due to 451 Timeout waiting for client input

Dear Expert,
   I don't know what cause our sendmail servers can't delvier email to some domains (Using hotmail / Yahoo mail can send and delvery). I try to monitor the mail log and always display "451 Timeout waiting for client input" this error message. Also, always wait a long long time until timeout and prompt the 451 error message.

   However, I try to use /usr/lib/sendmail -v [email protected] < test or telnet command to send the email. It is successfully delivered the mail.

   Our email servers mainly receive from external and relay internal mail to external one ( without open relay). So what I don't know what I do wrongly in the sendmail configuration.

   Please help to answer my question, many many thanks anyone attend this mail.

Log Info
220 xxx.xxx.xxx.xx Microsoft ESMTP MAIL Service, Version: 5.0.2195.6713 read
y at  Sat, 11 Oct 2003 18:38:09 +0800
>>> EHLO mailserver.xxx.xxx
250-mail.xxx.xxx.xxHello [xxx.xxx.xxx.xxx]
250-TURN
250-ATRN
250-SIZE
250-ETRN
250-PIPELINING
250-DSN
250-ENHANCEDSTATUSCODES
250-8bitmime
250-BINARYMIME
250-CHUNKING
250-VRFY
250-X-EXPS GSSAPI NTLM LOGIN
250-X-EXPS=LOGIN
250-AUTH GSSAPI NTLM LOGIN
250-AUTH=LOGIN
250-X-LINK2STATE
250-XEXCH50
250 OK
>>> MAIL From: SIZE=137592
250 2.1.0 [email protected] OK
>>> RCPT To:
>>> DATA
250 2.1.5 [email protected]
354 Start mail input; end with .

Then 451 timeout ...

Many Thanks for your professional advice

Answer : Can't delivery email due to 451 Timeout waiting for client input

i want to be sure that you understand the sequence of events that occurs in a mail transaction. Using the log messages in the question I may be able to make this a bit more clear, as follows:

Sender initiates the transaction and the receiver replies with:
     220 xxx.xxx.xxx.xx Microsoft ESMTP MAIL Service, Version: 5.0.2195.6713 ready at  Sat, 11 Oct 2003 18:38:09 +0800
Sender replies to the greeting with:
    >>> EHLO mailserver.xxx.xxx
Receiver replies with capabilities:
    250-mail.xxx.xxx.xxHello [xxx.xxx.xxx.xxx]
    250-TURN
    250-ATRN
    250-SIZE
    250-ETRN
    250-PIPELINING
    250-DSN
    250-ENHANCEDSTATUSCODES
    250-8bitmime
    250-BINARYMIME
    250-CHUNKING
    250-VRFY
    250-X-EXPS GSSAPI NTLM LOGIN
   250-X-EXPS=LOGIN
   250-AUTH GSSAPI NTLM LOGIN
   250-AUTH=LOGIN
   250-X-LINK2STATE
   250-XEXCH50
   250 OK (marks the end of the receiver's capabilities)
Sender issues the "Mail From":
    >>> MAIL From: SIZE=137592
Receiver replies:
250 2.1.0 [email protected] OK
Sender specifies the recipient: and that data is to follow:
    >>> RCPT To:
    >>> DATA
Receiver indicates the recipient is okay:
    250 2.1.5 [email protected]
Receiver states it ready to accept the message body:
    354 Start mail input; end with .
Sender starts sending data to the receiver and ends the message body with a '.' on a line by itself.

So in this case we can see that it is an exchange server that is to receive the message. And from what I hear you say it seems that your problems occur only when exchange is the receiver. Until we know from a sniffer trace if the message body is sent and terminated in the normal fashion we can't tell who is at fault.