Words of warning: Servers\Mail Users view in 8.5.1 Domino Directory has changed
I recently ran into a nasty surprise in an agent I wrote ages ago that uses the Servers\Mail Users view in the Domino directory to find users per server, which performs some checks against the mail database.
The agent loops through the entries, using the NotesViewEntry.ColumnValues instead of accessing the underlying document, in the view which worked fine all the way up to Domino 8.5.0
After upgrading the Administration server to 8.5.1 the agent started reporting problems with the database owner, ACL etc.
Turned out that the Name column of the view has been changed from displaying the user name in Abbreviated format to 'Lastname, Firstname'.
So if you've ever used this view in any of your code, check to see if this change has consequences, and while you're at it also check for changes to any of the other views in the Domino directory that might have changed. I know I will
Oh and what did I learn from this? - When writing code that relies on information from a system database, add an additional view (with the prohibit design refresh ... property set of course)