Mail acceptance failure, BDAT command

[This topic is intended to address a specific issue called out by the Exchange Server Analyzer Tool. You should apply it only to systems that have had the Exchange Server Analyzer Tool run against them and are experiencing that specific issue. The Exchange Server Analyzer Tool, available as a free download, remotely collects configuration data from each server in the topology and automatically analyzes the data. The resulting report details important configuration issues, potential problems, and nondefault product settings. By following these recommendations, you can achieve better performance, scalability, reliability, and uptime. For more information about the tool or to download the latest versions, see "Microsoft Exchange Analyzers" at]  

Topic Last Modified: 2006-08-21

The Microsoft® Exchange Server Analyzer Tool transmits the EHLO Extended Simple Mail Transfer Protocol (ESMTP) command verb to port 25 of a remote server and analyzes the response to determine whether the remote server supports the BDAT ESMTP verb.

If the response received from the remote server in reply to the BDAT command verb does not indicate a non-retrievable error, the Exchange Server Analyzer then transmits a test message to the remote server and analyzes the response to determine whether the message transfer was successful.

If the Exchange Server Analyzer does not receive the expected response string of '250', the Exchange Server Analyzer displays an error.

Exchange Server 2003 uses Exchange-specific extended SMTP verbs to communicate with other servers that run Exchange Server in the organization. It is more efficient to communicate with Exchange-specific extended SMTP verbs than by using standard SMTP. If support for the ESMTP command set is disabled, Exchange Server performance may be hindered.

The BDAT ESMTP command verb is an alternative to the DATA command. When an SMTP virtual server responds to the EHLO keyword with CHUNKING, the SMTP server indicates that it supports the BDAT command and will accept messages in chunks. This takes two arguments:

  • The first argument indicates the length of the binary data packet. Therefore the SMTP host does not have to continuously scan for the end of the data. The receiving server counts the bytes in the message. When the message size equals the value sent by the BDAT command, the server assumes that it has received all the message data.

  • The second argument indicates whether the data packet is the last packet in the current transmission. The second argument is optional.

This error indicates that although the BDAT ESMTP command verb is supported by the remote server, the subsequent BDAT mail acceptance test failed.

To resolve this error, check the messaging environment for issues or configurations that might be blocking the BDAT command verb such as:

  • Transient network outages

  • SMTP proxies

  • Firewalls

  • Applications

For more information about how to remove or restore the default support for these verbs, see Microsoft Knowledge Base article 257569 "How to turn off ESMTP verbs in Exchange 2000 Server and in Exchange Server 2003" (

For more information about how Exchange Server uses the ESMTP command verbs, see "How to Load Exchange SMTP Properly" in the Exchange Server Transport and Routing Guide (