1. Please check if you have set the smtp setting in web.config correctly.
<system.net>
<mailSettings>
<smtp from="no-reply@kooboo.com">
<network host="127.0.0.1"/>
</smtp>
</mailSettings>
</system.net>
2. Yes, you need to create a page plugin to send the mail. It is easy to do, inherit from IPagePlugin, upload the assembly under "Extensions\Plug-ins", assign the plugin to the View or Page.
3. In the lastest released version, the validation rule just can be do like that. Will provide some new helper method for that in the next version.
File Attachment(s):

EmailPageplugin.zip
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Regards,
Jifeng Huang
Kooboo Team
Microsoft ASP.NET MVP