When the recipient sends a Non-Delivery Report (NDR), they may well not include a reference to the original sent message. Additionally the NDR may be sent to Postmaster versus the original sender.
In-reply-to
is not a required value for bounced messages, there is no standard that is adhered to for bounced messages. Each provider is unique and it can be Null.
You would need to monitor inbound emails for NDRs.
Resources:
Can you please let us know why this email bounced?
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