Why Outlook 2007 converts incoming HTML email to plain text?

I use Outlook 2007 and the problem is following:

Sometimes (too often, but not always) I receive email in plain text format even though it was sent in HTML format.
(I have tested this with my friend who is also using Outlook 2007)

Also if I send HTML email to myself, I am often (almost always) receiving it in plain text.

I cannot isolate the reason for this (I am not an expert).

My preferred style is set to HTML. Note: I do receive html emails sometimes – so this problem is kind of random/mysterious to me.

How I could fix this annoying “feature”?

All help appreciated!!