In Tools Options Read, is read messages in plain
text checked? I don't know if that is the problem, as
even if 'plain text' is ticked, an attachment paperclip
should appear at the right of the message title bar.


No, he doesn't have that checked. I also read through the strings talking
about Rich Text and double-checked. I'm sending and he's receiving in HTML
format.

I might try sending him an email in plain text with a test attachment on it.
If that works, then I can set Outlook to only send to him in plain text.
Somehow I doubt that does it, but it's worth a try.

Thanks,
Mike