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Exchange not able to send some emails: NDR 550 5.4.300

Hello, my company is having issues sending emails to and

I made sure we're not listed on any spammer lists, I have an SPF and DMARC setup in my external DNS.   Below is the NDR that I got back.

I tried to send an email to, and this is what was returned to me.

Any idea's what is wrong?

Delivery has failed to these recipients or groups: (
Several attempts to deliver your message were unsuccessful and we stopped trying.
It could be a temporary situation. Try to send your message again later.

Diagnostic information for administrators:
Generating server:
Receiving server:
Total retry attempts: 193
12/16/2021 5:01:16 PM - Server at returned '550 5.4.300 Message expired -> 451 4.4.395 Target host responded with error. -> 421 4.4.1 Connection timed out'
12/16/2021 8:48:58 AM - Server at ( returned '451 4.4.395 Target host responded with error. -> 421 4.4.1 Connection timed out'
Original message headers:
Received: from ( by ( with Microsoft SMTP Server
 (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id
 15.2.922.13; Tue, 14 Dec 2021 09:11:25 -0800
Received: from ([::1]) by
 ([::1]) with mapi id 15.02.0922.013; Tue, 14 Dec 2021 09:11:25 -0800
From: Dan <>
To: "" <>
Subject: test 2
Thread-Topic: test 2
Thread-Index: AdfxDZLutegXFZ+wSNGmejtShJ8m5A==
Date: Tue, 14 Dec 2021 17:11:24 +0000
Message-ID: <>
Accept-Language: en-US
Content-Language: en-US
x-originating-ip: []
Content-Type: multipart/alternative;
MIME-Version: 1.0

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thanks for the quick reply. i updated the reverse PTR yesterday, and my ISP said it could take up to 24 hours to take effect.  today, I'm able to send emails to that domain.

I'll follow the first article you mentioned as soon as I get a chance, I'm out of the office now.