Microsoft Knowledge Base Article 262168 contains the following INTRODUCTION:
Microsoft Windows 2000 and Microsoft Windows Server 2003 include a Simple Mail Transfer Protocol (SMTP) service. The SMTP service can be used as a relay host. By default, this SMTP service advertises 8-bit Multipurpose Internet Mail Extensions, also known as 8BITMIME, and lets 8-bit MIME messages be relayed. Some products cannot handle 8-bit MIME messages. Products that cannot handle 8-bit MIME messages include the following:
|•||Microsoft Exchange Server version 4.0|
|•||Microsoft Exchange Server version 5.0|
|•||Microsoft Exchange Server version 5.5|
This article describes how to turn off the SMTP service so that it does not advertise 8BITMIME. You want to do this to prevent outside mail hosts from sending in 8-bit MIME messages.