You may not be able to.

The reality is that not everyone uses the same e-mail client software, and
each  "client" displays content in its own way, guided by whatever options
its user or their network administrator has chosen. That's the case even
when the e-mail format is "HTML" - you might imagine that is universal, but
in fact one of the objectives of HTML was that the user ought to be able to
have some control over how it is displayed. Also, it may be that your
e-mails have links or references that are local to your network, or are
otherwise restricted to users on your network. Thos e resources might be
images, templates (of one form or another) or perhaps other stuff I haven't
thought of.

What I might do is try to have a look at what one of your external
recipients is actually receiving, decoding what is in the message, and
working out what their system is likely to do with it. But in all
probability, what I would /actually/ do is try to distribute a plain text
e-mail with a succinct message and a link to the thing I wanted to display,
and put that thing up on my corporate website somewhere. That way, people
who really want to see it in all its glory will be able to do dso, and
people who don't will receive a short message that causes them minimal
distraction.

Peter Jamieson