Include .jar as a Soapui plugin

I followed this example http://blog.smartbear.com/sqc/creating-your-own-teststeps-in-soapui/ and now i'm dealing with the .jar.
i did the "Export as jar" in Eclipse but when i try to include it as a Soapui plugin I get the following error:
"com.eviware.soapui.plugins.MissingPluginClassException: No plugin class found in \mailplugin\soapui-emailteststep\target\soapui-emailteststep-plugin-1.0-plugin.jar"

Also i tried to do it with the mvn install command and i get the same error.

Can anyone explain how to make the jar to hold it as plugin, or tell me the structure of the .jar?
Thanks