External licenses include legacy portal licenses and Communities licenses, e2a can be used with 'Customer Portal Users' and 'Community Portal Users' licenses. Please see the steps below to configure e2a with external licenses:
Setup | Manage Users | Users | Your User
Installed Packages | Ortoo:Email-to-anything | Manage Licenses | Add Users
Setup | Customize | Communities | All Communities | Your Community | Manage | Administration | Members | Select Permission Sets

Setup | Customize | Communities | All Communities | Your Community | Manage | Administration | Tabs

Please contact us at support@ortooapps.com for any questions.