grails.plugin.greenmail.GreenMail is not in the same package as com.icegreen.greenmail.util.GreenMail.. thus stop() fails to find the super's services field at runtime as (being a Java class) it has package protection on the field.
It should call super akin to the start() method.
If you are able to submit a pull request I'll push out a new release for it.
I've created pull request that fix this problem gpc#10
I have a problem when the method stop() is called from an integration test in a Grails 3.1.4 application. The error said: "No such property services".
Anyone has the same problem? Is related to this?