To create a publicly available URL for the delivery reporting endpoint, you must configure a Salesforce Site. In the Setup menu search for "Sites" in the Quick Find box and open the Sites option under user Interface -> Sites and Domains -> Sites as shown in the following image. If you have not yet done so you should accept the Salesforce Sites Terms of Use and register your Salesforce Site Domain.
Once this has been accepted, click the "New" button under the Sites list to create a new Salesforce Site. You should provide a meaningful label and name for the site, choose a default web address to allow for more granular routing, set the site as active and choose an active site home page. As this site is only used for the endpoint we recommend using the "InMaintenance" visualforce page provided by Salesforce as standard. You should also set the Clickjack Protection Level to "Don't allow framing by any page (Most protection)". You can see an example configuration for a site call "Messente Delivery Reporting" in the following image.
Press "Save" and your site will be saved and provisioned as shown in the image below.
With your site now provisioned you can configure the Guest User permissions.