Actually, it won't.
As long as "Use Word to read RichText messages" is selected, the original
message will be spell-checked.
Thru all of this (and this has been posted in two places), it has been
checked, and no suggested solution has worked.
I just unchecked it, and Voila!
Apparently, as long as it was checked, if (1) I was using anything except
Plain Text as my outgoing format, and (2) if the original message was in
RichText, Word was assigned to read [and spell-check] the original message.
And we all know that PlainText does not allow the font formatting that I need.
So, my solution is to select HTML as my outgoing format, and to de-select
"Use Word to Read RichText.†This allows me to have full font formatting,
and for my selection of "do not spell-check original message in replies and
forwards" to be activated.
Whew!
Thanks for your prodding to make me re-test everything.