Setting up mail() in PHP for multiple domains

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http://serverfault.com – We have a Linux server with a single site set up on it (site1.com), down the road we may set up additional sites. When we send an email through php it comes from [email protected] We would like to change it to [email protected], but also make it so that when we add site2.com down the road, we can have the default email set to [email protected] for instance. I tried setting it in .htaccess, but that didn't seem to work. maybe I did it wrong. php_value force_sender [email protected] Linux noob, so what's the best way to do this? (HowTos)