Unfortunately, if I click the lightning bolt, it wants me to bind the "from" field to a dynamic database value. That's not what I want to do. I want to enter a designated email address.
I also tried clicking on the "From" button next to the "from" field, and that brings up a dialog box for changing the From Settings.
But any change I make there is ignored -- when I save the changes and save the modified form, the "from" value reverts back to the old setting (and the generated email shows a blank in the "from" field, as it did before).
I confirmed this by re-opening the saved form -- the changes I had made (and saved) in the "Create Email Message: From Settings" dialog have disappeared, and the field reverted back to what I didn't want.
I have run out of time to investigate this any further for my current client project. The inability to designate a "from" value is annoying, but at least for now, we can live with that limitation. Most of the other functionality seems to work okay.