Are you getting that error on non-question-and-answer threads?
If you are only getting it there, then I thought that we understood what was causing it? The question and answer thing tries to make you either comment on an answer, vote on it, or reply to the original question.
If you reply to a random email, you are likely replying to an existing answer. Reply to the start of the email thread at your end, and it’ll probably work.
Your challenge is that it likely isn’t clear in your email client which threads are in “question and answer” mode and which are not.
The good news is that almost no-one toggles the post voting flag per thread, so you can have a good guess about it based on the category.
FWIW I’m not defending the way that this restriction is implemented; just explaining what I thought we understood about how it works.
Edit: Yes, it does still seem to work exactly like this. This is an email reply to a non-post-voting thread, this is a reply to the original question of a post-voting thread, and when I tried to reply to an answer of a post voting thread I got this error:
We’re sorry, but your email message to [“replies+95941670d2ae5c4f4bb163834fdc155e@community.openstreetmap.org”] (titled Re: [Help and support] Download coordinates) didn’t work.
Reason:
You are not allowed to create a post in reply to another post.
If you can correct the problem, please try again.