I've got a main domain and 2 subdomains in our organisation. Exchange 2016 CU20 was installed in one subdomain. All was working perfectly! I've upgraded Exchange to CU21, no mistakes during upgrade! Everything was successfully! But when I got in ECP - the version of Exchange - was old (2242.4 CU20). Now have problems with new-made mailboxes! Constantly error in sending them emails - no such users! Before installing in main domain i've done PrepareSchema\AD\Domain; In subdomain, where exchange was - PrepareDomain. Looking Exchange Organisation Properties (ADSI Edit) - in main domain - old version (SerialNumber - Version 15.1 (Build 32242.8) In subdomain - new version - Version 15.1 (Build 32308.8) !! AD synhronisation is working! Wthat's the problem? And how to solve it !!!