How to configure apache / php / postfix website emails when using vhosts?

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http://askubuntu.com – I have a LAMP webserver configured to serve multiple websites. Each virtual host has various PHP applications, mainly Wordpress. When users sign up to the Wordpress sites, email is sent by PHP through to postfix, and then on to the receiver. The problem is that postfix is identifying itself to the remote server with the contents of /etc/hostname, which is not a fully qualified domain name. Some mail servers reject this and the mail bounces. Additionally, the return path is being set to one of the vitual host domains, seemingly at random. I could set /etc/hostname to one of the website domai (HowTos)