It is common for Internet Service Providers to periodically block SMTP connections to third party hosts. The best way to correct this solution is to use a port other than port 25 that requires authentication.
- First open the Exchange Management Shell
- Run the following command to find out what your current port is for your connector(s): Get-SendConnector | ft Id*,Sm*s,po*
- Change the port to an alternative port supplied by your Smart Host provider (more than likely 465 or 587, I recommend using the TLS port which is 587) by running this command: Set-SendConnector “Send Connector Identity” -port 465. You will of course change your send connector identity and port to match your configuration.
- In the Exchange Management Console, expand the Organization Configuration, select the Send Connectors tab, and double-click on your connector. Select the Network tab and click the Change… button. Confirm you are using Basic Authentication over TLS (if relevant to your configuration and port number) with the correct username and password.
- In the Exchange Manage Console, click on the Toolbox menu item
- Double-click on Queue Viewer and under the Queues tab, right-click on your delayed queue and select Retry.