OWA is not designed to function in a small channel window, and the best way to integrate Outlook would be to link to Outlook Web Access (OWA) from a bookmarks type channel.
Simulated Single Sign-on
Simulated Single Sign-on can be used to allow the user to browse from the portal to OWA if OWA is using Form Based Authentication. (Note: that Form Based Authentication is greyed out on a cluster server because FBA isn't available on a cluster.)
You'll need to make sure you have form based authentication enabled. see doc:
Customizing the Outlook Web Access Logon Page
You also need to use SSL or modify the registry to not use SSL
Finally, in your security context file, if you're using LDAP, make sure you have an LDAP Caching context set and that it is set above the LDAP context.
You also may be able to integrate with SPNEGO (essentially Kerberos over HTTP - Microsoft's latest attempt to integrate windows desktop with your browser) or NTLM authentication. Both may be best supported by IE on Windows and the web server (Apache or IIS) uPortal is run behind would need to use NTLM to authenticate users.
Microsoft is focusing its Outlook Web Access portal integration efforts into SharePoint and they're doing it in SharePoint's proprietary API (aka. WebParts).
Yash Technologies, a leading provider of enterprise portal solutions, announced the release of JSR 168 Compliant SyncEx Collaboration Portlets for use by enterprise customers worldwide. The new offering gives Enterprise Portal customers the ability to integrate Microsoft Exchange emails, appointments, contacts, tasks into their portals quickly and easily. http://www.javalobby.org/java/forums/t52678.html
For more information on the SyncEx Portlets for JSR 168 Compliant Portals and free evaluation software, visit http://www.SyncEx.com/.