Years ago I was unaware it was an issue, then I did it knowingly once or twice but got slapped down for it, but it's been a very long time since I did anything like touch the Subject line in any way. I don't know anything about how threading works but if there's something I should keep an eye on I will. BTW there's a perhaps related issue: I almost always get two copies of every post, one will have the option of 'reply to list' the other: 'reply to all'. I'm in the habit of automatically deleting the second -- which, now that I think about it tend to come first. I'd prefer to only get one copy. Also, if I reply to the one instead of the other, something changes but honestly I haven't bothered to pay any attention.I agree. What I meant to refer to (but didn't make clear, my bad) was changing the subject when the topic is NOT new. Ray has done that several times in this thread, but Bart is surely correct about this being Thunderbird's fault. (The possibility didn't occur to me.)