[snip]
>FWIW, the Department of Defense has banned HTML email as a security
risk.

...And the DoD users universally revile this security measure.  :)  I
get that security and usability are tradeoffs.  There comes a point when
your users begin to opt-out of your technology "solutions", and this one
is rapidly approaching that.  Good security can in and of itself be bad
security, if you can believe that.