mail() and Sender: field

Hi All I'm having a problem using the mail() function of PHP. It seems to vary between platforms (ie. this problem doesn't not occur on my Win32 development platform, but does on my Linux production server). I'm sending mail using the mail() function and including a "From:" and "Reply-To:" header. Usually this is sufficient to ensure that receipients will see that the mail is from the desired address I included in the From: header. However on my Linux server (PHP 4.3.2/Sendmail 8.11.6), something is automatically adding a "Sender: " field which is populated with the address of the apache user (apache@my.hostname) -- so this appears to the e-mail recipient as From: apache@my.hostname ON BEHALF OF [the name/address i specified in From] Any idea what is causing this, and how I might be able to disable it? Adam