Error 'Content is not allowed in prolog' in Outbound emails

Salesforce will consider outbound messages delivered successfully only if the target endpoint returns the response in the below format.
 

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/">
<soapenv:Body>
<notificationsResponse xmlns="http://soap.sforce.com/2005/09/outbound">
<Ack>true</Ack>
</notificationsResponse>
</soapenv:Body>
</soapenv:Envelope>

The response must conform to the following format:

• The response must be a SOAP envelope using the XML namespace: `<REDACTED>
• The SOAP Body must contain a notificationsResponse element in the namespace `
• The notificationsResponse element must contain an <Ack>true</Ack> element

If the response does not match this format — for example, if it contains plain text, HTML, or malformed XML before the SOAP envelope — the "Content is not allowed in prolog" error is returned. This commonly occurs when the endpoint returns an HTTP error page or redirects before the SOAP response body.
Related error: If the endpoint returns <Ack>false</Ack>, the Delivery Failure Reason will instead read: "SOAP response was a nack."

Steps to resolve:

1. Review the target endpoint's response to verify it returns a valid SOAP envelope with <Ack>true</Ack>.
2. Check for any HTTP redirect, authentication challenge, or error page returned before the SOAP body.
3. Update the endpoint's response handling to return the correct SOAP acknowledgment format.