Mail Delivery System <[email protected]> emails

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I periodically get emails structured in the following manner. I have changed the first part of the email address and the IP address for anonymity purposes: This message was created automatically by mail delivery software. A message that you sent has not yet been delivered to one or more of its recipients after more than 24 hours on the queue on The message identifier is: 1bKRrp-002dEE-1v The subject of the message is: RE: AJA Mailing Address The date of the message is: Tue, 5 Jul 2016 08:02:26 -0700 The address to which the message has not yet been delivered is: [email protected] host [] Delay reason: SMTP error from remote mail server after end of data: 421 4.2.1 : (DYN:T1) No action is required on your part. Delivery attempts will continue for some time, and this warning may be repeated at intervals if the message remains undelivered. Eventually the mail delivery software will give up, and when that happens, the message will be returned to you. Ultimately I receive an email that the email was not sent & informing me of such. When I actually make contact with the person, either using another one of their email addresses or another one of mine, they indicate they do receive my email. There is no consistency with this problem. It's just very annoying. Is this a legitimate email from or Mail Delivery System which is the actual sender of the email?
Hello, Thank you for your question regarding email delivery issues. Often, AOL rejects mail due to DMARC policy, ensure you have Setup DMARC records correctly. Since this is just a delayed delivery notification, it has not failed yet. The link AOL provides lists the following possible reasons: "The IP address you are sending from has been temporarily rate limited because it is not Whitelisted, unexpected increase in volume, or poor IP reputation." The final email stating that it "was not sent" should provide a more detailed error. If you can provide it, we are happy to help you review the cause. If you do not want to post it here in our public forums, you can Our Bounceback Parser tool may also help you determine the cause. If you have any further questions, feel free to post them below. Thank you, John-Paul