X-Authentication-Warning: serverx.mysite.com: apache set sender to webmaster@yoursite.com using -f
The reason is that you haven't told your mail server to trust apache as a sender. To fix it, edit the file /etc/mail/trusted-users by adding apache to the list.
Reference: Fix for X-Authentication-Warning - HowtoForge Forums | HowtoForge - Linux Howtos and Tutorials.