Office 365 Hybrid Emails Stuck in Queue: target host responded 4188.8.131.52 connection timed out mail-onmicroosft.mail.protecion.outlook.com.
You receive the following error after running the Hybrid Configuration wizard, all emails are building up in the Exchange Queue.
To resolve this issue, follow the below steps.
Make sure that the all your exchange servers are added to the Send connector, if they are not added you can run the following in PowerShell to add the servers.
Set-SendConnector “Outbound to Office 365” -SourceTransportServers EX01,EX02
Next, if the issue is not fixed enable “Proxy through client access server” on the “Outbound to Office 365” send connector.
Adding the “Proxy through client access server”, has resolved my issue of mail being stuck in the Exchange On-Prem queue.