-I reply to it, the sent message is stored in my Sent folder but I can view the whole conversation in a thread

Hi I am trying to configure Thunderbird on my Mac to allow me to delete a whole conversation, including replies/sent messages, when I click 'delete'. I have installed the Conversations add-on and have the tick box entitled 'Toolbar Button' checked, which seems to suggest that if I click 'delete' on a conversation, the whole lot will be deleted, regardless of the folders that each separate message is in. What I would like to happen is: - I get an email, it appears in my inbox. -I reply to it, the sent message is stored in my Sent folder but I can view the whole conversation in a thread - The conversation continues, with sent messages being saved but appearing as a conversation (all the above is working fine) - Then I come to delete the conversation. I want with one click to be able to delete all the messages, including those in the Sent box and elsewhere. The add-on suggests that it should do this but it isn't working. Could it be something to do with how I have things configured? Eg how Thunderbird files my Sent items? I've achieved similar in other Apps, such as Mac Mail and Postbox, by creating a Smart Mailbox that displays the contents of several folders. Can anyone please help? Many thanks, Si