Dear Benjamin,

if you receive an email, the format belongs to the "sender", so you can not
change this format for this incomming email.

If you replay to an e-mail, it is also the format of the sender which sent
you an email. You could change the format manualy. Open the email => Format
=> Plain Text).

I you create a new e-mail you can prepare the contact to send an e-mail to
this contact only in plain text:
http://email.about.com/od/outlooktips/qt/et010406.htm
http://support.microsoft.com/kb/290809/en-us

Maybe it helps.

--
Oliver Vukovics
Share your Outlook PST files without Exchange: Public ShareFolder
Synchronisation for Outlook and Exchange: Public SyncTool
http://www.publicshareware.com