You can use the SMTP command EHLO to determine which capabilities an SMTP mail server supports. First, use the Telnet command to connect to port 25 on the server. Open a command prompt and type

telnet <servername> 25

To query the server, type


This command returns a list of keywords representing the extensions that the server supports. (For example, the EHLO keyword for the Chunking extension is CHUNKING; the keyword for the 8bit-MIMEtransport extension is 8BITMIME. Each extension's Internet Engineering Task Force—IETF—Request for Comments—RFC—defines its EHLO keyword.) Microsoft Exchange Server 5.5 supports a smaller set of Extended SMTP (ESMTP) verbs than Exchange 2000 Server does. Thus, large messages travel more slowly between an Exchange 2000 server and an Exchange 5.5 machine than between two Exchange 2000 servers.

The discrepancy in Exchange 2000's and Exchange 5.5's SMTP support causes a problem in a mixed-mode organization when you want to use an SMTP connector to link an Exchange 2000 administrative or routing group (which Exchange 5.5 views as a site) and an Exchange 5.5 site. In this scenario, Exchange 2000 issues an EHLO command to determine the verbs that the Exchange 5.5 Internet Mail Service (IMS) supports. The IMS doesn't support BDAT, so the Exchange 2000 machine uses the traditional DATA verb to send messages. The use of the DATA verb presents no problem with nonsystem messages, which the sending server encodes in Transport Neutral Encapsulation Format (TNEF). However, Exchange 2000 encodes system messages in Summary TNEF (S-TNEF), a form of binary MIME that isn't well suited to the DATA verb. Therefore, system messages don't transmit successfully.

To fix the problem, you need to make a registry change that forces the Exchange 2000 server to translate S-TNEF messages to TNEF before sending them. (The Microsoft article "XCON: Required Registry Setting When Using SMTP Connector as a Site Connector to Exchange Server 5.5" at describes this solution.) Another solution is to replace the SMTP connector with an X.400 connector running over TCP/IP.

To resolve incompatibilities and enable connections between Exchange 2000 and another SMTP server, you might need to turn off individual ESMTP verbs. The Microsoft article "XFOR: How to Turn Off ESMTP Verbs in Exchange 2000 Server" ( explains how to use the Microsoft Management Console (MMC) ADSI Edit snap-in to disable specific verbs.