When people reply to a Google+ post of mine or mention me, I get an email. On at least one of those emails, I was able to reply via email. That doesn't seem to be working any more and now messages say "you can't reply to this email". Is there a way to restore this functionality?
You should be able to reply to the comment right within the email message.
More at the official announcement: http://googleplus-update.blogspot.com/2012/05/now-reply-to-google-plus-comments-from.html