When you try to create a mailbox and you see this error:

New-Mailbox : ExchangeGuid is mandatory on UserMailbox. Property Name: ExchangeGuid

Make sure that the “Rus Agent” is running.

Get-CmdletExtensionAgent | ? {$_.name -like "*rus agent*"}
Enable-CmdletExtensionAgent "RUS agent"

The Rus Agent adds the missing attributes in this case your ExchangeGuid to your AD account!