We seem to get a fair amount of questions and cases where the supportability of certain OWA customizations isn’t clear.  In the past, many organizations have found ingenuous ways to modify the OWA files on the server in order to make them behave in different ways.  Unfortunately, changing these files takes you into uncharted territory.  The OWA web application undergoes extensive testing, and when Microsoft makes any changed to these files, it has to go through testing again.  If you modify a javascript or aspx file, then you are running OWA in an untested and unsupported configuration.

Now, many companies have gone outside of the “supported” realm and made these types of changes anyway, and hopefully they do their own testing to make sure that they aren’t introducing errors.  Unfortunately this isn’t quite enough.  Prior to Exchange Server 2007 SP1 RU5, the update installer would not update files that had been altered.  This meant that if you made a change to a file such as logon.aspx, the file wouldn’t get replaced with the latest version.  For many installations, this behavior simply meant that the customizations would “stick” through the update.  The note above about being in an untested configuration still applies, and you have the added consequence of negating any testing that you did when you initially made the change.  Things can get worse though.  There were some changes made to OWA with SP1 RU3 that depend on an updated logon.aspx file.  If you had made changes to that file prior to SP1 RU3 and you apply RU3 or RU4, OWA ceases to function.  This behavior is noted in the release notes for SP1 RU3 and SP1 RU4, and documented in KB 956582Exchange Server 2007 SP1 RU5 changed this behavior to always replace files even if they have been changed, so that an update doesn’t cause OWA to stop working, but this means that modified files can be replaced.

So, what are the supported ways to customize OWA?  The short answer is:  “If you can find documentation about how to do it on MSDN or Technet, then it is supported.”

Here’s some information on the supported scenarios:

Outlook Web Access Customization Architecture – Overview of how OWA Customization works.

Outlook Web Access User Interface Customization – You can add elements to the navigation bar and the “New” dropdown.

Outlook Web Access Forms Registry – You can register new custom forms to be used for specific item classes (Note: this only works for the default mail folder view in OWA, and won’t work for other folders like the Calendar).

How to Create a Theme for Outlook Web Access – You can customize the look and feel of the OWA client, including the logon and logoff pages*. 

* A special note for branding of the logon and logoff pages:  These changes are limited to the image files and the logon.css file in the Base theme directory.  This is necessary because without a logged in session, the server has no way to check what theme or experience (Basic or Premium) it should use.  Updates to Exchange create a new version-based folder for themes and the modified files should be copied forward. You should plan to test logon and logoff screen customizations after an update to make sure that the non-modified files haven’t changed in a way that causes a problem with your customizations.