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I send a lot of emails through mailmerge and Outlook but only 40 at a time. Recently I had over 150 returned with 553 envelope sender rejected OR 550 - 5.1.0 address rejected. AND they had treid over 20 times to deliver the message.
Usually they will bounce straight back if the address is faulty.
Anyone have thjie problem?
I also had this undeliverable problem, but just with one email, and I posted about it under 'infuriating email failure' recently.