In fact, a default policy is preconfigured to allow both plain text and HTML content. You will need to modify the Default SMTP Policy to accomplish what you are trying to do. On the Message Format tab of the default policy, configure MIME to Provide message body as plain text. The problem is that this will only affect "Internet" mail. That is mail sent to or received from the Internet (a.k.a. domains outside the scope of your organization).
IMAP4 and POP3 clients can be configured to use plain text only from an Exchange server for all e-mail, but a MAPI client cannot. You might want to investigate ways in which you can automate the modification of each users profile as an alternative.
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