Send bulk emails without timeout

I am developing a plugin that sends emails out to users. I am having difficulty in finding the most performance enhanced way to send these emails out and prevent a server timeout. In other words, I would like my plugin to scale, so if someone that uses it on a site with a hundred thousand users, my plugin will still work. Perhaps there is a way to execute each email or two as a separate script that gets executed one after another? Some help/insight on this would be appreciated. Thanks