Is it possible to send mail notification from Splunk Free Edition using custom REST point?
Splunk free has no search scheduling therefore no alerting.
You can test if it can send email form a search, with the search command |sendmail
and build your own scheduler, with the CLI and cron/OS scheduler.
http://docs.splunk.com/Documentation/Splunk/4.3.4/SearchReference/Sendemail
Not mentioning that you can also pay for Splunk Enterprise...