Triggering a background process is pretty easy, here's some script ideas to do it:

http://de2.php.net/manual/en/function.exec.php#35731

http://stackoverflow.com/questions/1019867/is-there-a-way-to...

Alternatively, you can use curl to trigger a request inside of your page to another page (send_email.php) and don't wait for the response.

http://tech-hacks.net/tech/13/creating-php-cronjobs-without-...