How to Fix Exchange Server “421 4.3.2 Service not available”

When I examined the receive connector logs to identify the problem with the mail traffic I was experiencing on one of my Exchange servers, I saw that the error “421 4.3.2 Service not available” was constantly repeated.

When I followed the mail flow in the log, I observed that the steps continued successfully, but at the last stage it gave the error “421 4.3.2 Service not available“.

auth login,
334 authentication response,
SMTPSubmit SMTAccept,
235 2.7.0 Authentication Successful,
Mail From : <…@…>,
421 4.3.2 Service not available,

I checked the ServerComponentState of the server, and saw that the HubTransport was Draining.

Get-ServerComponentState (hostname)

To fix this situation, we need to use the following command.

Set-ServerComponentState ExchServerName -Component HubTransport -State Active -Requester Maintenance

Check again Get-ServerComponentState (hostname)

Have a nice day !