To fix the 405, you likely need to contact your host. This is potentially something that will come up in other circumstances so that's worth doing. These sort of processes require a number of browser refreshes. This may be triggering an aggressive anti-DoS tool they're running. If it's an option, admin.php and deferred.php should be whitelisted.
The alternative is to remove the email sending task, but I'm not sure if that's what you want. You should be able to cancel the job with the button when it is running, though you can also remove it from the database directly. If that's what you'd prefer, let me know.
