There's actually nothing that prevents that, if you craft the right headers you can reply to a thread you were not included in, and have it show up as a reply in the thread of common clients (tested Gmail and Outlook).

We added this feature at my $dayjob and I was quite surprised there is no authentication. But thinking about it, this is how mailing lists work (you aren't explicitly specified in "To:") so it makes sense you can do this.