As mentioned in this answer, it is supposed to be possible to respond to Facebook wall posts via email, by replying to the notification message.
This used to work for me, but now suddenly doesn't. I've tried both my normal email client and the Gmail web interface, but the result is the same: from my end it looks like the message went through, but what I wrote doesn't appear as a comment on the Facebook post, and there is no delivery failure notification or anything like that. The notifications in question do contain the text "Reply to this email to comment on this post".
How can I troubleshoot this? Is there any non-obvious requirement to these email replies that I'm missing?
EDIT: I discovered that I can in fact reply to private messages by email, just not to wall posts.