create mailbox in exchange 2010 for sub domain user

  In root domain controller, insert Exchange 2010 installation CD to d drive then type  d:\setup /preparedomain:xxx.yyy.zzz (your sub-domain)

The Name on the security certificate is invalid or does not match the name of the site

In the CAS role of Exchange Server, Open Exchange Management Shell, input the following:

1. Set-ClientAccessServer -Identity "servername" –AutodiscoverServiceInternalURI https://owa.exmple.com/autodiscover/autodiscover.xml 
2. Set-WebServicesVirtualDirectory -Identity "servername\EWS (Default Web Site)" –InternalUrl  https://nlb.nwtraders.msft/EWS/Exchange.asmx 
3. Set-OABVirtualDirectory -Identity “servername\OAB (Default Web Site)” -InternalURL https://owa.exmple.com/OAB 
4. Enable-OutlookAnywhere -Server servername -ExternalHostname “owa.example.com” -ClientAuthenticationMethod “NTLM” 
5. Set-ActiveSyncVirtualDirectory -Identity “servername\Microsoft-Server-ActiveSync (Default Web Site)” -InternalURL https://owa.exmple.com/Microsoft-Server-Activesync