I noticed that a couple of members changed the subject line when replying to a post via email, and the forum basically ignored the change.
I started looking into implementing a change for that behavior. You can follow progress at Changing email subject line should start a new post - feature - Discourse Meta
In the meanwhile, if you want to create a new post or change the subject line, please create a new topic by just emailing, e.g., email@example.com, rather than replying to a post. If you are just trying to reply, and aren’t trying to create a new thread, just hit reply.