We all can get a little lazy when it comes to emails, such as using the same subject line that has nothing to do with the most recent email. Beware of long threads. Some are necessary when related to a project; they are solid proof of agreements, disagreements and next steps. But some can be problematic.
A few years ago I was making dinner plans with two friends. The emails went on and on with last minute schedule changes, cancellations and even illness. As I scrolled through the week old thread, I came upon one email between my other two friends but I was copied all the same. One mentioned to the other that he was over me regarding a personal experience I was going through. He had washed his hands of me (his words). The other friend didn’t respond, if he did, he was smart enough to reply directly without dragging me into it.
If it’s not business, there’s no need to continue to send the same “used” email. There is no cost for beginning a new one. I held onto that email for a while. I never confronted him, in the end I pressed delete and washed my hands.