Thanks, Mark. As usual, your reply was rapid and helpful. I infer that you don't see the instability that my subscriber does. Do (can?) email providers passing the posts to the subscriber alter the headers in ways that might lead to that unstable behavior? Or can it occur because of the way T-bird tries to help the user associate display names with email addresses--autofill and address book automation? Do you have those kind of features turned off in your T-bird? Or is it more likely that the subscriber makes some mistakes that get interpreted as needing a change in From displays? Because only one of my subscribers is complaining regularly, it's not very important for me to do anything, but I'm still curious about the origin of this behavior.