why are my mail messages suddenly being rejected as spam?

Question

Diagnostic information for administrators:
Generating server: PH8PR20MB6366.namprd20.prod.outlook.com
******@gmail.com
Remote server returned '550 5.7.520 Message blocked because it contains content identified as spam. AS(4810)'
Original message headers:
Received: from MN0PR20MB4571.namprd20.prod.outlook.com (2603:10b6:208:372::7)
 by PH8PR20MB6366.namprd20.prod.outlook.com (2603:10b6:510:25a::15) with
 Microsoft SMTP Server (version=TLS1_2,
 cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.6277.30; Fri, 7 Apr
 2023 04:25:49 +0000
Received: from MN0PR20MB4571.namprd20.prod.outlook.com
 ([fe80::df50:83aa:18ce:267e]) by MN0PR20MB4571.namprd20.prod.outlook.com
 ([fe80::df50:83aa:18ce:267e%7]) with mapi id 15.20.6277.031; Fri, 7 Apr 2023
 04:25:49 +0000
Content-Type: application/ms-tnef; name="winmail.dat"
Content-Transfer-Encoding: binary
From: Gordon Myers <******@hotmail.com>
To: Andrew Pearce <******@gmail.com>
Subject: Re: N Sgr 2023
Thread-Topic: N Sgr 2023
Thread-Index: AdlllldCRY8KnenBQ52Xi9HobPUAcAAA1gSAAADJw4AAAHTMgAAMpvKAAASlsJAAyIEogAAAvywa
X-MS-Exchange-MessageSentRepresentingType: 1
Date: Fri, 7 Apr 2023 04:25:49 +0000
Message-ID: <******@MN0PR20MB4571.namprd20.prod.outlook.com>
References: <******@MN0PR20MB4571.namprd20.prod.outlook.com>
 <******@telenet.be>
 <******@telenet.be>
 <******@MN0PR20MB4571.namprd20.prod.outlook.com>
 <******@mail.gmail.com>
 <******@MN0PR20MB4571.namprd20.prod.outlook.com>
 <******@mail.gmail.com>
In-Reply-To: <******@mail.gmail.com>
Accept-Language: en-US
Content-Language: en-US
X-MS-Has-Attach: yes
X-MS-TNEF-Correlator: <******@MN0PR20MB4571.namprd20.prod.outlook.com>
MIME-Version: 1.0
X-TMN: [vjnJYowH+chNPpiYt0mUTE1COBZBtTjS]
X-MS-PublicTrafficType: Email
X-MS-TrafficTypeDiagnostic: MN0PR20MB4571:EE_|PH8PR20MB6366:EE_
Return-Path: ******@hotmail.com
X-MS-Office365-Filtering-Correlation-Id: 1b7f398b-9479-4204-7b7f-08db37202a61
X-Microsoft-Antispam: BCL:0;
X-Microsoft-Antispam-Message-Info:
	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
X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-MessageData-0:
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Answer 1

That's hard to tell, as the NDR message is generated based on the response on recipient's side. Usually, I'd advice you to check with an admin on their side, run a trace, etc, but that would not be possible in the case of a gmail address.

Instead, try minimizing the number of links, images and attachments in the message, clean up the text where possible, use a more descriptive subject. If you can contact the recipient by any other means, ask them to add your address to their "safe senders" list.

UPDATE: there seems to be an issue with the service as detailed below. Please wait until the issue is resolved, and report back if you are seeing similar behavior afterwards.