Emails not sending. marked as pending

Hello, the subscribe via email plugin does not appear to be sending out emails.

If I look in the emails log they are marked as pending but do not appear in the queue. For example please see - http://principal.roseberryacademy.org/wp-admin/admin.php?page=sbe-sent-mails

WPMUDEV support access has been enabled for this site.

It seems to be a similar issue as has been reported here - https://premium.wpmudev.org/forums/topic/emails-are-pending-and-are-not-sent-out-in-a-timely-manor

Thanks
Jack