An e-mail message from a regular sender, to which I have often replied, including a week ago, suddenly end up in the junk folder. It is quite possible that during that week I have tagged one or two rather different messages as junk, but isn't it rather obvious that mail from regular senders to which one regularly replies should not be junked unless specifically tagged as junk by the recipient? Such an erratic junk filter behavior makes Thunderbird a rather unreliable mail client.