Unfortunately you'll have to talk to your hosting provider.... most likely they have "safe-mode" enabled on the server:
Safe mode is a deprecated functionality that won't even exist in the next release of php. Because emails to large lists can take a long time, it will be necessary to be able to extend the script timeout to send out the emails effectively.
You will have to talk to you hosting provider to enable that setting (turn off safe mode). If they won't do it, then I'd suggest switching hosting providers.