User receives blank email

Server: MS Exchange 5.5 sp4
OS: nt4.0 sp6a
client: Outlook2000

Recently, my email users receive only blank email from external

From the header of the attached message, all the content is inside but can’t be displayed. It is a html format message.

Does exchange server support MIME? Could it be due to some mis-configuration of my exchange server or something wrong with the message?

Anyone can advise?