Had an issue, when after authenticating to the NetScaler, and then hitting the Email tab, The OWA login page came up.
Even though OWA worked fine internally, and an Internal version of the Access Gateway worked fine too.
Watching a network capture showed that it kept trying to use my local machine account, and since my desktop at home was not part of the domain, it kept failing using “MyDesktop\domainuser”
To Fix it, I switched from using SAMAccountName (me) to UserPrincipalName (firstname.lastname@example.org) in my “LDAP Authentication Server”, that way the domain was included with the login token, and passed on to OWA.
That was it, now OWA works internally and extenally!!