Emails are not being sent outbound, when configured to relay through the Email Security Service (ESS). Cannot connect to the ESS infrastructure to relay outbound emails.
The most likely source for this kind of errors is the configuration of the network and security devices that lie between the email server and the ESS infrastructure.
On premises type email server configuration, going out through a single public IP. Not using a public cloud based email hosting solution (Office 365, Google Apps).