If you could, please forward me an email that shows the full headers (likely have to toggle a setting on your email client to see all headers) and I'll take a peek. You can copy & paste the headers into a PM if you like, but make sure they are the full headers and not just the from/to/reply-to lines.
What it used to look like for me (no sender):
non working mail.jpg
What it looks like for me now (sender exists):
working mail.jpg
I haven't seen one without a sender field since I played with it, but it sounds like you still are...