I have Postfix set up on a Linux box.  Its only job is to send emails that are automatically generated through cron jobs (Perl scripts) and PHP web pages to various domain email address.  I'm not sure I'm explaining this correctly, but the Linux box sits on my intranet and sends emails out to the internet.  However - because my domain hosting company uses sender authentication, the emails are being bounced because, even though I set a valid FROM address in my scripts, Postfix is sending them from root@domain.com or apache2@domain.com, etc.  I have figured out how to set the domain name correctly in main.cf (mydomain=domain.com), but I can't figure out how to force it to send from a specific user and not the user that's running the script.   I'd like to set Postfix to send all emails from validuser@domain.com regardless of which cron job runs the script.  I hope I explained this correctly.

ANY HELP WOULD BE GREATLY APPRECIATED.