ERROR:Updating hybrid configuration failed with error 'Subtask ValidateConfiguration execution failed: Configure Mail Flow

Scenario
You need to add a new domain to the Hybrid Configuration Wizard. You complete the following correct steps.

• Add Domain to O365 and Verify
• Add Domain to On-Premise Exchange as Authoritative
• Run the HCW and Complete Domain Verification etc...
• At the end the HCW Completes with error:

ERROR:Updating hybrid configuration failed with error 'Subtask ValidateConfiguration execution failed: Configure Mail Flow

A Closer look at the HCW logs may include an error something like:

ERROR:System.Management.Automation.RemoteException: This operation is not required. Organization is already enabled for customization.

Resolution
In my case, there was no rhyme or reason for the HCW to error. So after trawling many Office 365 Blogs and community forums, also having a go at removing connectors as per one MS KB  made no difference (Which is just bad advice anway in my opinion,) here just for ref:

 The following simple steps worked for me.

 1. Connect to your Exchange On-Line via Power shell
     (So a key point here is that you need internet access to Office 365 from the EMC/EMS)

•  $UserCredential = Get-Credential
• $Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential $UserCredential -Authentication Basic -AllowRedirection
• Import-PSSession $Session 

2. Run the following Command against one of your SMTP Domains (Ensure it completes ok)

• Get-FederationInformation -domain mysmtpdomain.com

3. Re-Run HCW. 
                               
                             

All good !!!!  Bizarre, but it works. Hope this helps