cron jobs with membership plugin


I thought i read earlier that cron jobs on wordpress only run when a user logs onto the site.

I am just wondering as I have created some renewal notification emails (under communications in the membership plugin), but none of the emails have been sent. I am able to send myself emails, so emails are working on the site but i am confused why no emails have gone out.

Any ideas? I am working on a site where there are a few hundred members awaiting an annual renewal email. A speedy reply would be much appreciated.

Many thanks