Email can be sent as plain, html or both... the ones you can't view are
the html only ones... it's my experience that these are normally spam,
thoughtless people or companies ;-)

In five years with this, I've been able to address it by requesting that
those who sent me html only mail to set my email address to "plain"
which many email clients offer or use "both"

That said, another option is to see if your email provider offers a "web
mail" interface... if so, in most cases, you can go to that site using
pie and read html only emails.

Beverly Howard [MS MVP-Mobile Devices]