OT: Did you do anything special when replying? I noticed that this is the second reply that breaks threading and I'm not sure how that happens. I also tried to reproduce it without success.

I just clicked on reply as a non logged in user (obviously ;-) ). Thanks for this fast fix, about the SMTP settings. Unfortunately the link I got does not seem to work.

Ok, got it. The issue occurs when posting failed and the message is then
sent again.