To setup email alerts on the 3945L running FreeNAS, first log into the web GUI and go to System > Email. Here you will enter your email server logon details.
Any alert emails will be sent to the root user. In order to receive these emails, an email address must be set for the root user. To do this, go to Account > Users > View Users and select the root user from the list.
With the root user selected, click the Change E-mail button at the bottom of the page and you will be prompt for an email address.
Once everything is configured, you may test your settings by going to System > Email and clicking the “Send Test Email” button at the bottom of the page.