You use the Email Security.cloud Service and see attempts to connect to your server from a server named mlort.messagelabs.net with the IP 184.108.40.206.
We regularly test our clients' routes to ensure that they are not operating in an Open Relay state.
Your server should be locked down to the IP Ranges for the service. Therefore, your email servers should not accept connections from the server that sends these test emails. If your server proves to be an Open Relay or it has been compromised, our technical support team will notify you of the issue.
For your reference, the details you may see in the test email are: