Extreme incoming email delay or undeliverable

For example, I got the following at 10:02 AM MST today (2023-08-12):

Diagnostic information for administrators:

Generating server: DS7PR19MB5830.namprd19.prod.outlook.com
Total retry attempts: 15

<username>@cox.net
Remote server returned '550 5.4.300 Message expired -> 451 4.2.0 cxr-ibgw-6007a.stratus.cloudmark.com 40.107.92.124 AUP#??'


Original message headers:

Received: from DS7PR19MB6280.namprd19.prod.outlook.com (2603:10b6:8:97::5) by  DS7PR19MB5830.namprd19.prod.outlook.com (2603:10b6:8:79::6) with Microsoft  SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id  15.20.6678.22; Sat, 12 Aug 2023 05:03:32 +0000
ARC-Seal: i=1; a=rsa-sha256; s=arcselector9901; d=microsoft.com; cv=none;  b=BA83jxhGb9b3SuenARBe2INGke2xPpUaWbRUbQVWi7fc4dGS/LNEX4817Th9m1oZXGcg/KsR05LgpmHQLKsvbmLZIGykyd7CJRHz/5cf9RKQwh75vkrpPJkYkmPDo0bhCRXa7Byk+WLfDYkx8+aubu7MNC9Upkod4cxlJqIBgFyuK2AyIvOpny0Cyp/a427TAxIDZP7z28pfu4B5i3Ci+Q7PTexuhi+P1VreESolpABqP2QYIpD0nwjJx7fupGu1xYhTmSjupjezxLfHqHhfe++h6enXnK0qPGkfr4BhqmXX/kjhIShAH8sRVnUGUxfiioPyB5fhp6n+GInAtlmHxQ==
ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=microsoft.com;  s=arcselector9901;  h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-AntiSpam-MessageData-ChunkCount:X-MS-Exchange-AntiSpam-MessageData-0:X-MS-Exchange-AntiSpam-MessageData-1;
 bh=XGrNlzDf0BThZLuP5i6Lg2hs3/LOasnbFhzrQT0xeTI=;
 b=DGOwlV28LkOVJi8+6Ble++tvTZmrGGotF5dg8tlqblmS9UjeNDQ5WekKOXtWi6fsEzWsigPcW6yVyp+xS3PAc4CcULYPm4ujiQXBPVv21R4/xyu7anUrWfg+XE2yLDiCrVWMkEejlmiIoefPNBFLZeaMScwUymQwAz3fy559hMD+spEOGjRkmbLmWdcrWFFloASiPMYV7PJP5/5T2bt9R9OSUWAwdVA2wn+gItQMgfGa85CyMtIsYSIg23PFadcG/IUTpBqAYQJ+BoQFDs/+e6c4sejvoNRnxv2aqAoka6xms7OlvByqWaO1qiNZ1YQVbEzFpJTuB68G8E+7hFQcFQ==
ARC-Authentication-Results: i=1; mx.microsoft.com 1; spf=pass  smtp.mailfrom=arizona.edu; dmarc=pass action=none header.from=arizona.edu;  dkim=pass header.d=arizona.edu; arc=none
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=arizona.edu;  s=selector1;  h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck;
 bh=XGrNlzDf0BThZLuP5i6Lg2hs3/LOasnbFhzrQT0xeTI=;
 b=tCLGghR5jRVHRWWRY8rgc99unxpoH4tPqdqUhvmugVlWtC1fTVazUrumpKJSHVQuirXKylZsZ1uqaqX0jL/OO7ROYr8EcTkEIt9Oi4szVS8v6SOGkNcLOI/mfGWTxSAG+wfSE/nY9z1a9erHBzuOByQ5cSFueoX3rIwsPeD92pY=
Received: from BYAPR19MB2919.namprd19.prod.outlook.com (2603:10b6:a03:b4::26)  by DS7PR19MB6280.namprd19.prod.outlook.com (2603:10b6:8:97::5) with Microsoft  SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id  15.20.6652.30; Fri, 11 Aug 2023 17:01:27 +0000
Received: from BYAPR19MB2919.namprd19.prod.outlook.com
 ([fe80::95fc:e3b0:22d7:a686]) by BYAPR19MB2919.namprd19.prod.outlook.com
 ([fe80::95fc:e3b0:22d7:a686%4]) with mapi id 15.20.6652.029; Fri, 11 Aug 2023
 17:01:27 +0000
From: <userrname@arizona.edu>
To:  <username@cox.net>
Date: Fri, 11 Aug 2023 17:01:27 +0000

From here;

If you are having problems with email delivery send us an email at unblock.request@cox.net for assistance. Your email must contain all of the following information in order for us to process your request.

First name
Last name
Email address
Phone number
Domain
IP/CIDR range of outgoing email servers

From the following choices, indicate the item that best describes the issue you are experiencing.
My IP address is being blocked
My domain is being blocked
My email is being marked as spam
My email is not arriving to the intended recipients
My email is being delayed
Other

Include a detailed description of the issue you are experiencing, such as the date and time when the problem began and the corrective action that has been taken.

Include email server logs related to the issues.

I'm seeing the same problem with mail my spouse is sending to me.  Different Exchange server, but the same error message.  Most messages don't go through, and the few that do take 20+ hours for a simple message with no attachments.

I'm also seeing the same issue.