[exim] Exim sending empty outbound sender

Hi,
I have an exim installation I am trying to get up and running, working
off the default configuration. All outbound e-mail originating at that
server tries to deliver messages without any sender e-mail address. (I
am not talking about bounces.)
I've debugged the attempted delivery, and sure enough it will send a
command as:
MAIL FROM: <>
When I look at the e-mail frozen in the queue, the headers only show
the display name of the sender, without an e-mail address, such as:
From: root
I'm guessing there is some configuration line I'm missing. I've tried
sending from command line 'mail' and other gui applications, all with
the same result. Anyone seen this and know the resolution off hand? I
don't have the configuration file at my disposal as I write this
e-mail, but can get it.
-Brian