As these are server settings they must be made by your Exchange administrator using the Exchange System Manager snap-in. If your admin is unfamiliar with the necessary setting, I've provided links to the relevant Microsoft documentation.
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Description of the method for setting plain text as default for OWA...
<http://support.microsoft.com/default.aspx?scid=kb;en-us;319252&sd=tech
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Troubleshooting guide if plain text setting has been made but html is still outgoing default...
<http://support.microsoft.com/default.aspx?scid=kb;EN-US;297464
>
...
.d.