XCON: Link States Continually Change and Event ID 451 and Reset Event Messages Are Logged in the Network Trace

You may experience the following symptoms:

�	Mail flow is sporadic between servers.
�	Links states continually change (the states go up and down) between servers that are known to have good links. If you either use a ping command, initiate a telnet session, or send a message to test the links, these tests succeed.
�	If you run Network Monitor to create a network trace, a large number of 451 events (which indicate "Timeout waiting for client input") and reset events may be logged from the server that is receiving the mail.

NOTE: This article assumes that you understand how to use Network Monitor, network traces, and Simple Mail Transfer Protocol (SMTP).

Typically, if you run a network trace, you note that the receiving server sends a "250 2.6.1" return code to the originating server after the sender successfully completes the DATA portion of the message. In the trace, the last lines of this 250 return code resemble the following data (this data is displayed in the network trace hexadecimal data):

51 75 65 75 65 64 20 6D 61 69 6C 20 66 6F 72	Queued mail for
20 64 65 6C 69 76 65 72 79 0D 0A	delivery

NOTE: Review the last four hexadecimal values in your trace. If these values match the values that are displayed in the preceding table, this the article does not apply to you.

The last hexadecimal values (0D 0A) represent a carriage return and line feed. These values indicate that the 250 return code is complete and that the server is acting correctly.

The symptoms that are described in the "Symptoms" section of this article occur because the MTS-ID message that is returned with the 250 return code is greater than the buffer size that is allocated for the return string (the buffer size is 1024 kilobytes [KB]). If this behavior occurs, the server that sends the 250 return code does not send the carriage return and line feed codes (0D 0A), and a "250 2.6.1" return code is not created. As a result the sender is not notified that the message was successfully received. The connection times out while the server that sent the message is waiting for the 250 return code, and a 451 packet is sent back by the receiving server. As a result the connection is marked as "down" (a link state problem), messages get stuck in the queue, and non-delivery reports (NDRs) are sent to the senders.

To resolve this problem, install Windows 2000 SP2. The file that is responsible for this problem is Smtpsvc.dll.

Microsoft has confirmed that this is a problem in Microsoft Exchange 2000 Server.

Typically, the packet size of the problem 250 return packet is exactly 1024 KB. The following data is an example of the communication between the problem sender (Server A) and the receiver (Server B): For additional information about this problem, click the article number below to view the article in the Microsoft Knowledge Base: