I was sent an email to someone as part of an ongoing thread of emails in a conversation.

I drafted a reply, but felt a bit unsure so I sent it to someone I trust for them to look at first. Initially, the draft had been made by hitting the reply button to the original sender, but I removed them from the to: field and very thoroughly checked they were indeed removed before sending my potential response to this third party. The 3rd party replied to me and for a second my heart stopped because their reply was in the same conversation thread and it looked a lot like the whole exchange between them and myself was now broadcast to the very person I was trying to carefully word my reply to. Making it look even more like that was the case, was that the signature of the person who sent me the original I wanted to reply to was at the bottom of the third party's response to me along with their comments. What seems to have happened is that because I pressed the 'reply' button, despite changing the sender, it still included the whole thread of emails until that point. Which makes sense but was very much not what I was expecting since I don't recall seeing that there in the email before sending it to the 3rd party. I suppose it must have been.

Anyway, it seems to have been alright, there's no trace I can find that the exchange between myself and the third party has made its way back to the person that sent me the messages in the first place. But it's very messy and scary that this, to my mind, separate, conversation thread is now all wrapped in to the same one. It seems like begging for trouble even if I dodged it this time. It's my fault for making it a reply instead of a separately composed message to the 3rd party, but can I manually move those messages out of the thread in mail so I don't see them there in future?