[Exim] Config Problem: Unprivileged user [nobody@blah.com]

I have just started playing with Exim and have now installed
it as a replacement for sendmail on one of my live servers.

When I was using Sendmail I had it configured so that PHP
scripts on my server (PHP running as nobody) could send e-mails
with valid from lines. Since I installed exim I get all mails
from PHP as "Unprivileged user [nobody@???]".

Presumably there is an easy way to fix this - my aim is to
enable the e-mails to appear to come from the person who
typed their name in the from e-mail address on the form or
from a sensible address from the site.

Any pointers would be welcomed - I tried wading through the
documentation but I could really do with a quick fix.

Regards

Simon Clayton
Reference Technology Ltd