If you are looking at upgrading to RU12 for Exchange 2010 SP3 you might want to hold off and go directly to RU13. There is at least one bug that I have found related to OWA.
Shortly after upgrading to RU12 I received a complaint that URLs weren’t working if they were forwarded from OWA to a non-OWA user. After doing some testing I was able to verify the issue.
Microsoft uses a redir.aspx file to rewrite URLs in OWA. For example, if you receive an email that has a link to https://ehloexchange.com in OWA the URL will look like https://owadomain.com/owa/redir.aspx?REF=slHepgB0lKZqZ8dEKXILt9MpdmvI7gD1rtMw3FmjmuPTrs3erVAFodHRwczovL2FrYS5tcy9xdGV4MGw. If you open the email through Outlook the URL will remain the original URL and you will not notice the bug.
As long as you are using OWA the URL will work correctly. This is a standard functionality of OWA and I expect to see the URL being rewritten. The issue I am seeing come when you try to forward an email from OWA to an external user. In RU11 and earlier, the URL would be formatted back to the original URL and the link would work correctly. After installing RU12, the link remains the rewritten OWA formatted URL even after leaving the organization. So when someone outside the organization tries to open the link, they are redirected to OWA to log in.
This has been confirmed by MS to be a bug with RU12. And in a Reddit post, they have said that the newly released RU13 will resolve this issue. This fix has not yet been listed in the KB article for RU13 and I have not had a chance to test if RU13 does resolve this issue. I will update this post once I’ve had a chance to verify the fix.
I have finally had a chance to test RU13 in my test environment. After installing the update I was able to verify that it has indeed resolved the OWA url issue I described above.