Según se dice en la web de OWA éste no es una API y por tanto no puede ser utilizado en un desarrollo personalizado, si bien se podría utilizar httprequest y httpresponse para interactuar con él.
Mi opinión es que estando tan bien implementado Outlook Web Access y lo conseguido de su interfaz, la única integración con DotNetNuke que se me ocurre sería la de compartir el inicio de sesión, es decir que el inicio de sesión en dnn, también sirva para loguearse en OWA, cosa que podría lograrse usando la autenticación ActiveDirectory...
Edito: enlace de interés
Development Criteria
| Criteria |
Outlook Web Access |
| Languages and Tools |
Outlook Web Access components are only available by means of the HTTPS GET and POST methods. |
| Managed Implementation |
Outlook Web Access is not an API and does not provide components for use in a managed or unmanaged development environment. However, you can use the HTTPRequest and HTTPResponse managed classes to access Outlook Web Access programmatically. |
| Scriptable |
It is not possible to directly access Outlook Web Access components by using a script. It is possible to use an HTTPRequest or similar object to retrieve information from Outlook Web Access. |
| Test/Debug Tools |
No tools are available for debugging calls to Outlook Web Access, because the internal architecture is not available for debugging, and the URL calls required to access Outlook Web Access programmatically are not documented or supported. |
| Expert Availability |
Outlook Web Access customization and component reuse is not supported by Microsoft. |
| Available Information |
Little or no information is available about customizing Outlook Web Access or reusing Outlook Web Access components and commands. |
| Developer / Deployment Licensing |
Outlook Web Access customization and component reuse is not supported by Microsoft. |