How can a listener verify the source of an outbound message?

To verify the source of an outbound message, checking the Salesforce Org ID included in the message is a reliable method. The Salesforce Org ID is a unique identifier associated with every Salesforce organization. When an outbound message is sent from Salesforce, it includes this Org ID as part of the message payload. By examining this Org ID, the listener can confirm that the message originated from a legitimate Salesforce instance. This adds a layer of validation, ensuring that the communication is coming from an expected and trusted source.

Other options may not effectively confirm the source of the message. For instance, while checking the message's timestamp could provide some context regarding the timing of the message, it does not authenticate the origin. Similarly, requiring user credentials could complicate the message transmission and might not be feasible for automated processes, whereas validating message size does not inherently confirm the source of the message but merely checks the dimensions of the data. Thus, verifying the Salesforce Org ID is the most effective method among the provided options for establishing trust in the message source.