"you've changed this message" how do i stop that from happening?

[BHills]

I keep getting a popup when previewing my digitally signed emails saying,
"you've changed this message. If you save the changes the message will no
longer be digitally signed. Do you want to save your changes"

I know there is a way to stop this from happening, I've done it before, but
I cannot for the life of me figure out how to do it again on my new portable.

Any help would be greatly appreciated.

Thank you!