It might happen that there are several mails in the local queue on the appliance - mails which have been created due to a misconfiguration. In this case it might be useful to be able to clear the queue.
The mail queue can be deleted via the page Manage email in the Web Configuration. On this page you can delete the emails one by one if you like.
Additionally, you can delete the emails using SSH. You need to enable root access via SSH and to connect to the device as root user. After login you can execute the mailq command to take a look at the mail queue, and rm /var/spool/nullmailer/queue/* to delete all mails from the queue.
Starting with Firmware 1.7.0, we use Postfix. So the last paragraph needs to be extended. But instead of the rm ... command, we need postsuper -d ALL. So maybe something like "Note: since firmware 1.7.0 the appliance uses Postfix as it's MTA. To drop the mail queue in this case, run postsuper -d ALL as root."