after checked the exchange logs, we found that exchange ANTISPAM agent was rejecting to these bounce/undelivered/rejected emails, and now we are getting these bounce back mails after added PMG IP address on exchange transport rules policy to bypass SPAM check detections.
we are not getting NDR reports for bounced/rejected/undelivered emails to internal sender, if we checked the logs it's showing that our email was rejected due to invalid address of remote recipient or IP was on blacklist, but users didn't get any bounce emails.
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