Ionel, I am currently unable to replicate this behavior in 16.  Can you check whether or not the user(s) exhibiting this behavior have been fully indexed?  If the indexing logs set to detailed, are there any exceptions?  Additionally, are there any console errors in the browser console window when you perform the sort?

Logging for Indexing was set to Exceptions only and I had nothing there. Changed it now to Detailed and will update after indexing is done. I manually set to re-index the user that reported the issue, as well as my mailbox (which seems to also be affected) and another user`s that I asked to confirm the behavior (at this point probably all are affected).

Nothing is logged into the browser`s console when sorting email in WebMail, apparently. 

Re-indexing the user apparently solves the problem.

If it helps, here is the log for the user that reported this issue initially, since I manually initiated the re-indexing of his mailbox: Indexing_Log.txt

Question: how can I tell if all users have been indexed or not (please do not tell me to look for all of them in the logs)? Is there an indicator in the user list, or can you implement one so that administrators can see the information?

If I look at Troubleshooting -> Mailbox Indexing, there is always 1-2 users that appear briefly there, but I just did this test: watched for a user to appear there and waited for his account to disappear. Then asked the user to sort items by Flagged and the same problem appears - only items flagged in the past day appear. Here is this user`s indexing log (did not yet initiate a manual re-indexing): Indexing_Log-2.txt

So now I know that manually re-indexing will solve the issue, but I do not know who needs this or not. Probably all that I did not manually re-indexed? And how can I queue them all for re-indexing, but limit the number of users being manually re-indexed at a time so as to keep resource usage low on the server?

Thank you!