[Exim] Strange source address

Hi,

This is my first time posting to this list and I'm pretty new to exim so
please bare with me :)

I have a website which uses one of our mailservers (running exim4) to send a
confirmation email to new users who sign up at the site. This is all working
great however when the users recieve this, the "From:" field shows

www-data [www-data@???]; on behalf of;
webmaster@???

Any ideas? Thanks in advance!

Dave