Microsoft has released Exchange 2016 CU7 and Exchange 2013 CU18. Have a look at the Exchange versions page for more information and links to the Microsoft KB articles and downloads.
In the eventviewer on an Exchange 2013 you might notice an error being reported every 60 seconds with Event ID 1121, MSExchange Replication Service.
Log Name: Application
Source: MSExchange Mailbox Replication
Date: 26-9-2017 11:43:14
Event ID: 1121
Task Category: Request
The Microsoft Exchange Mailbox Replication service was unable to process a request due to an unexpected error.
Request GUID: '4c037387-462c-446f-855d-1a4a9f4769bc'
Database GUID: 'f76ac2d8-bf20-4646-88a1-5f9161f5f312'
Error: Database 'f5c17f94-0b61-472e-9908-048b145e740c' doesn't exist..
Check if there are any open mailbox move requests, export requests, import requests of migration batches. If there are any open requests remove or complete these if there are no …
Continue reading EventID 1121 – MSExchange Mailbox Replication FIX