For reference.

Strip HTML email in Outlook into plain text Content: First, this is secure as many of the worms and bugs rely on HTML script code. One good example could be the needless advertisements or images sent inside spam (junk) emails. When you so much as view an email inside your email software, the senders webserver gets a timestamp of you having accessed the image. This of course does not happen with plain text, because there’s no image, so there is no inadvertent access?.

Second, it is also a bit faster to download and view email that doesn’t have all the unnecessary frills of HTML email (tables, bold, italics etc).

Start | Run | regedit Find this key: HKEY_CURRENT_USER\Software\Microsoft\Office\
10.0\Outlook\Options\Mail On the Edit menu, point to New, and then click DWord Value. With the new Dword value selected, type ReadAsPlain. Double-click the new value to open it. In the Value Data box, type 1, and then click OK. Click OK, and then quit Registry Editor. Just to be sure, close Outlook and restart it. From now on, all your HTML email messages will show up as simple text. After you turn on the Read as Plain Text? feature, users notice the following changes:

The changes are applied to the preview pane and open messages. Pictures become attachments to avoid loss. Digitally signed messages are not affected.