For the Help and support section, we are not allowed to post replies to other replies (at least it doesn’t work via email). One gets the following message:
Because it’s specific to help and support, the error is coming from the Q&A plugin, likely because of the differences in the model and comments. It’s possible the email message is coming from the plugin, or from Discourse itself. In either case, I reviewed the plugin settings and admin settings and there’s nothing relevant, so this is an upstream issue.
I think the bigger upstream issue is that this message is confusing, and it’s not clear what you did wrong.
My impression was it happens when you reply to a reply in the Q&A-category. I get it frequently. You must reply to the original question in order to have your email going through. I believe this is a bug, because on the website you can add “comments” to answers, and this is what the Q&A plugin should do for contributions via email in reply to a reply.
Anyway, if this cannot be solved easily it would already be quite helpful to have the text of the notice adjusted (quoting the full text one has tried to send, and adding a link to the topic).
It seems to be related to the Q&A plugin in as much as I’ve only seen the message in those threads, and it’s not a “deterministic” error in that if I save the reply text elsewhere, diacard the Discourse reply, reply again and paste in exactly the same text it works just fine.
I had thought that qoting may be involved, and maybe voting up a previous answer, but the fact that it also occurs by email (where there is by definition less context) menas that that is less likely.