XFOR: Disabling 8-BIT MIME Support on the Windows NT Option Pack SMTP Service

Exchange Server administrators may notice the following error message: This may be a result of a mail host attempting to send 8-bit MIME messages to the Exchange Server computer. The Microsoft Windows NT Option Pack comes with an SMTP service that can be used as a relay host between the Internet and Exchange Server. By default, this SMTP service advertises and allows 8- bit MIME messages to be relayed. This article describes how to prevent outside mail hosts from sending in 8-bit MIME messages by disabling the advertisement of 8BITMIME.

To make the Windows NT 4.0 SMTP service stop advertising 8BITMIME, you need to alter the metabase key that tells it to advertise 8BITMIME. Use the Mdutil.exe utility that is included on the Windows NT Option Pack CD.

NOTE: The Mdutil.exe utility is not installed with the Option Pack by default.

To turn the 8BITMIME advertising off, set the advertisement of 8BITMIME to 0 by typing the following command at a command prompt: NOTE: The above command is shown on two lines for convenience, but it should be typed as one continuous command. You may need to stop and restart the SMTPSVC for this option to take effect. To verify that disabling or enabling 8BITMIME has taken effect, do the following:

1. From a command prompt, type the following command:
   telnet localhost 25
   After it connects, you should see a banner response with "ESMTP spoken here."
2. On the Terminal menu, click Preferences, verify that the Local Echo check box is selected, and then click OK.
3. In the Telnet window type the following command:
   EHLO yourdomain.com
   Where yourdomain.com is your e-mail domain, and then press the ENTER key. You should then receive a response of 250- and various options. If you disabled the 8BITMIME, it should no longer be listed.
4. Type quit in the Telnet window to close the session, and then close the Telnet window.

The response should be: The number at the beginning of the returned line, 36865, is the property ID number for 8BITMIME. The hexadecimal value (0x0)={0} means that 8BITMIME advertising is turned off.

To turn MIME broadcasting back on, execute the above command after replacing -value:0 with -value:1. This sets the parameter value to 1, which turns MIME broadcasting on.