When you run the Hybrid Configuration wizard to set up a hybrid deployment between your on-premises Exchange Server 2010 Service Pack 3 (SP3) environment and Exchange Online, you receive the following error message:
'Subtask ValidateConfiguration execution failed: Configure Mail Flow
This issue occurs if hybrid mail flow connectors already exist in the on-premises environment and in Exchange Online.
Important You will lose mail flow while you perform these steps. To minimize the disruption, perform this procedure during a scheduled downtime.
Remove the hybrid mail flow connectors from the on-premises environment and from Exchange Online. To do this, follow these steps:
Remove the Send and Receive connectors from the on-premises environment. To do this, open the Exchange Management Console, and then follow these steps:
To remove the Send connector, expand Organization Configuration, click Hub Transport, click the Send Connectors tab, right-click Outbound to Office 365, and then click Remove.
To remove the Receive connector, expand Server Configuration, click Hub Transport, click the Receive Connectors tab, select the server that has the Receive connector that you want to remove (usually the hybrid server), right-click Inbound from Office 365, and then click Remove.
At a command prompt, run the following command to sync all domain controllers:
Remove the Send and Receive connectors from Exchange Online. To do this, follow these steps:
Sign in to the Office 365 portal, click Admin, and then click Exchange to open the Exchange admin center.
Click mail flow, and then click connectors.
To remove the Send connector, click Hybrid Mail Flow Outbound Connector under Outbound Connectors, and then click Delete ().
To remove the Receive connector, click Hybrid Mail Flow Inbound Connector under Inbound Connectors, and then click Delete ().