Phishing email reports returned as Undelivered Mail Returned to Sender

Recently the phishing and scam emails I periodically bundle and forward as attachments to reportphishing@apple.com and reportphishing@apwg.org are all bouncing back to me as "Undelivered Mail Returned to Sender."

Does anyone know why this would be happening? Previously these went through fine - I even received “Thank you for your report” responses.