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I recently went through a migration of Exchange 2010 to Exchange 2016. One of the changes to the environment was the move to putting Exchange behind our SAML login platform ADFS 3.0. One idea we had was to let our OWA users know right on the website. Microsoft does not give a way to do this through supported methods by modifying the CSS. As we are moving off of Exchange we didn’t mind putting that in. I believe the only real reason for this is to prevent issues when you install service packs or updates that reset this page. However once you get exchange 2016 installed in your environment you can no longer install updates to the 2010 environment so no need to worry there.
The default path of the file you need to modify is:
C:\Program Files\Microsoft\Exchange Server\V14\ClientAccess\Owa\auth\logon.aspx
Make a backup of this file in case you mess something up, like I said Microsoft does not support changing this file so you need to be able to back out.
Open logon.aspx in a text editor and find the following line.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"><html>
<head>
Below that last line give yourself some extra spacing to work by hitting enter 4 times so you don’t get confused when you copy\paste.
Paste the below HTML and modify to your own environment.
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You will need to do this on all 2010 CAS servers in your environment.
Hit save and go view your OWA logon page, it should look similar to below.
You now have another method of letting your users know the page will look different and hopefully cut down on the amount of calls into the help desk.
I would always suggest in addition to this using email and some other notification to the users like a company newsletter to get the word out there.