[Internal] Setting a Unique Custom DLR Callback URL with Each Request

We have a custom DLR callback feature that is not currently documented on our website which might be helpful. Using this parameter you can specify a unique callback URL in each message request. You can simply specify a unique callback URL with each request by adding parameter &callback=<url> to your HTTP message requests. If this parameter is omitted, DLR callbacks will default to the Callback URL for DLRs set for your account (which is why it is required in any case).