XCLN: Client SMTP Connectivity Using Secure Socket Layer

When you send SMTP messages from an Internet Mail Service-enabled client to an Exchange Server computer, you may want these documents sent in a secure fashion. When you configure an e-mail client, for example Outlook Express, to communicate by means of Secure Socket Layer (SSL) for this reason, you may receive the following error message: This indicates that the client was unable to connect to the server.

There will not be any event logged on the Exchange Server computer.

Microsoft clients configured to send SMTP message by means of SSL are not able to communicate correctly with the Exchange Server computer. When SSL is enabled on an e-mail client, the communication port defaults to port 465, defined by Netscape. When the client issues an SMTP command to send a message, the Internet Mail Service does not receive the communication because it is monitoring port 25.

If you change the port within the client to use port 25, and you try to resend the message, Exchange Server still will not respond because the client is issuing SSL commands to the server, while the server only understands Transport Layer Security (TLS) command sets. Because the two computers are speaking a different security protocol, they will not be able to negotiate a session.

At this time, SSL-enabled Microsoft Internet mail clients send security command requests (SSL) that are incompatible with the Exchange Server Internet Mail Service security command set (TLS). Consequently, there is no way of using this configuration for sending SMTP secure mail. Outlook Express 5, as well as Outlook 2000, will include support for TLS over port 25. Both of these products will support the TLS command set.

There are a two workarounds that will work until the Outlook Express 5 and Outlook 2000 products are released.

• Communicate with your Exchange Server computer through Point-To-Point Tunneling Protocol (PPTP), which will encapsulate all messages sent to or received from the server. For additional information, please see the following article in the Microsoft Knowledge Base:
  180979 OL97: Using Outlook with a PPTP Connection
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• Use message encryption by means of certificates to send and receive messages securely and discretely.

Additional information on the following subjects can be found at the following locations:

TLS

Message Encryption