Accessing OWA using alternate UPN

Our internal domain name is sdt.local, but we want users to be able to log onto OWA using username@stratadecision.com, our primary external domain name.  Historically users have logged onto OWA using sdt\username.  I followed the instructions inhttp://technet.microsoft.com/en-us/library/cc772007.aspx to add stratadecision.com as an alternative UPN suffix for the domain, then followed http://mikefrobbins.com/2010/09/02/change-owa-logon-format-to-upn-exchange-2007-2010/ to ensure the OWA logon page would prompt for email address rather than domain\username.

I and a couple of other users found we were unable to log onto OWA using our primary email addresses, though username@sdt.local and sdt\username would both work.

In ADUC, I noticed user accounts now show @sdt.local and @stratadecision.com as drop-down options for "User logon name" on the "Account" tab of the user properties dialog, with @sdt.local being the default.  I found that by switching the drop-down selection from @sdt.local to @stratadecision.com, users were able to log onto OWA using their email addresses.  Interestingly, even after switching that drop-down selection back to @sdt.local for a given user, the user still was able to log on with his email address.

Not being sure of the full implications of changing that drop-down selection, we elected not to do it for all users.  And although we're a company of fewer than 100 people, I don't relish the idea of switching the drop-down selection, ensuring the user logs onto OWA with his email address, switching the drop-down selection back to @sdt.local, and confirming the user can still log on with his email address -- 85+ times.

What's the fix here?

Thanks in advance.

Shawn