Add dependency to plone.api

setup.py

@@ -42,6 +42,7 @@ setup(name='collective.dms.mailcontent',
       zip_safe=False,
       install_requires=[
           'setuptools',
+          'plone.api',
           'plone.app.dexterity',
           'plone.directives.form',
           'collective.contact.core',

src/collective/dms/mailcontent/configure.zcml

@@ -16,6 +16,7 @@
 
   <i18n:registerTranslations directory="locales" />
 
+  <include package="plone.api" />
   <include package="plone.app.dexterity" />
   <include package="plone.supermodel" />
   <include package="plone.app.relationfield" />