How to fix [Message=421 4.3.2 Service not available] error?

421 4.3.2 Service not available error.png

Recently we have faced an issue in Office 365 test account where we cannot send email from our Office 365 test account to our On-Premises Exchange Server.  Note: Our On-Premises Exchange Server is a Hybrid Exchange Server. We also have receive connector for the Barracuda. Message trace in Exchange Online Admin: Reason: [{LED=450 4.4.317 Cannot connect […]