[Exim] Changing "Sender" header

I have Exim installed as the MTA on a web server that hosts several virtual
domains (Apache 1.3). Almost all of those domains are using some sort of
PHP-based CMS software. Most of the CMS packages provide some ability to
send email from a web page. For instance, one script allows a user to
recommend the site to another person. When the recommendation fields are
completed and the "Send" button is selected, the PHP script sends an email
message to the intended recipient.

My problem is this... The header of email generated by PHP scripts on my
server *always* shows the Sender as "nobody@www". I've grown to suspect
that much of the mail sent from my server using these kinds of scripts gets
filtered out as spam.

I want the header info of email generated by a PHP script to change
depending on the virtual domain that generates the request. Is there an
simple way to do this? If you're kind enough to respond to this question,
please go easy... I'm fresh to Exim.