OT: A mailman question: how can I reply to a thread that's created before I join the alias?

[Max (Weijun) Wang]

Hi All

While browsing the mail archive of an alias I'm not a member of, I  
find a quite interesting thread. However, even if I join the alias  
immediately, I cannot receive the thread. Is there any chance I can  
reply to that thread now? Or, I have to wait for someone else to first  
reply to it?

Thanks
Max