Outgoing email bouncing

Question

HELP!

Emails I have tried to send over the web recently have been bounced back to me. This doesn't seem to happen when I'm replying to someone, but has occurred even to frequently emailed contacts. One recent rejection notice said:

Your message wasn't delivered because the recipient's email provider rejected it.

Diagnostic information for administrators:

Generating server: CO6PR08MB7802.namprd08.prod.outlook.com

******@proton.me Remote server returned '554 5.7.0 < #5.7.520 smtp;550 5.7.520 Message blocked because it contains content identified as spam. AS(4810)>'

Original message headers:

Received: from CY4PR08MB3382.namprd08.prod.outlook.com (2603:10b6:910:80::22)
 by CO6PR08MB7802.namprd08.prod.outlook.com (2603:10b6:303:146::16) with
 Microsoft SMTP Server (version=TLS1_2,
 cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.9700.17; Mon, 16 Mar
 2026 14:43:46 +0000
Received: from CY4PR08MB3382.namprd08.prod.outlook.com
 ([fe80::fee5:159b:3f0:c16f]) by CY4PR08MB3382.namprd08.prod.outlook.com
 ([fe80::fee5:159b:3f0:c16f%5]) with mapi id 15.20.9632.019; Mon, 16 Mar 2026
 14:43:46 +0000
Content-Type: application/ms-tnef; name="winmail.dat"
Content-Transfer-Encoding: base64 
From: ...
To: ...
Subject: Classified Thread-Topic: Classified
Thread-Index: AQHctVFAvjjWJzQSLEGiHbQQsfLPPw==
X-MS-Exchange-MessageSentRepresentingType: 1
Date: Mon, 16 Mar 2026 14:43:46 +0000
Message-ID: <******@CY4PR08MB3382.namprd08.prod.outlook.com>
Accept-Language: en-US
Content-Language: en-US
X-MS-Has-Attach:
X-MS-TNEF-Correlator: <******@CY4PR08MB3382.namprd08.prod.outlook.com>
msip_labels:
MIME-Version: 1.0
X-MS-PublicTrafficType: Email
X-MS-TrafficTypeDiagnostic: CY4PR08MB3382:EE_|CO6PR08MB7802:EE_
Return-Path: ...
X-MS-Office365-Filtering-Correlation-Id: cd93673d-a908-4f64-9ed9-08de836a6da7
X-Microsoft-Antispam:
 BCL:0;ARA:14566002|31061999003|9400799043|20031999003|24071999003|39105399006|15030799006|37011999003|19110799012|12121999013|15080799012|12050799012|8062599012|461199028|8060799015|25031999004|40105399003|19061999003|37111999003;
X-Microsoft-Antispam-Message-Info:
 =?us-ascii?Q?IFfhQf4FVdSdynznkVwBDISudP0xIPDYAkEincB0EGGnctkPrYsVclGIv/eh?=
 =?us-ascii?Q?BAcIOzakcSqBdYOZV0kRtmZ4DADycsgz6sSsRsd7rPYWeLOjqsqvf1kA1vyc?=
 =?us-ascii?Q?pfNbc5BuTg1sGIe0J+BJBiE6JTbnVoeQj7yHODrheNNvugDYaiuPAEDQprKm?=
 =?us-ascii?Q?m6+kN+Li5oha8zgPNrSfZJwbAqWZEgSi8HIL3iUCuZ7m6ehJ+D4EPkf7Pvql?=
 =?us-ascii?Q?rcqgsLdaO1odEHdcm0/FlNhC9Lz+i64MeGs82CUXW7bmMfqAOJpqaXxq8kdA?=
 =?us-ascii?Q?9YYPafRFYZPA3qBC5EwqLCMBA35u4+EUO/v2n1kODhszX/kKNI7bcKIN7JoS?=
 =?us-ascii?Q?xwlnnJDhFm1epG1atqFqEIJ+rHzzU10ck8OL2c/WAIBUMbZuqfk0DpGVxWGf?=
 =?us-ascii?Q?25qo5q+Bw9bhkAfdbevbfgrwGvK4x2bUzMjy7JSpBGO1UjYQowzoUPFniFlx?=
 =?us-ascii?Q?nO3iHwYWQ20638dztSolAGVi5fmLoNjImX/Gj0CWsEJ/U3LWksaGMnqs6NZK?=
 =?us-ascii?Q?oRVVuwNBOAqBhHiSSG56S77H1REZg5edik15xGgUP0mhkYUxd0ga9OJwh5AF?=
 =?us-ascii?Q?SfWh+uPrDgCVXfm2GVspWMsE1a0dAkYMPVAA5UvaDa7DN2S/2oRU9lVo1ODi?=
 =?us-ascii?Q?+/kENeNR9kUS2b/RefkJkEs1hOlBF76Y565dX8Fu9W61sB2ojbeS9B6sqXKg?=
 =?us-ascii?Q?dcnyqy2Bo1W8Cra8ECmP7f0ybou6mVprC42dKKNI/F/ZBvGv9Xj1ySs3ofCD?=
 =?us-ascii?Q?PbWXKYbkAWUc4cQHJNe75iItLYjKXB2b05QO1R26urvQx9vKu0SOsc9F9fJX?=
 =?us-ascii?Q?UesbSna4UrUSVIOIqAHnoIW8LwmOlcMOJPh7muAG0SvDIV1GJxNwgDrhhpcq?=
 =?us-ascii?Q?EIWFcvXhowIa95aHooHjMf8ow21ONpewFUDwLxUs2l40h8MwDsujmeu2CnIW?=
 =?us-ascii?Q?9Ukj9k/3/k7et4xUCp9glJjUkVRwjRQkLwRAXhSBjFtssLZXXURhgX0gRPS6?=
 =?us-ascii?Q?6sgVP8m6HzH/5Vnr4/waK+FKuUZExiQnLULXQpf/udUq7KVfi9gPYthaElnz?=
 =?us-ascii?Q?OLDDDq+GCZOXF32jAuHN83l+9sQ67QC0wR/Anc5YWVJg1wd51hFZTBOeeybi?=
 =?us-ascii?Q?hy8KNYcqAm/WmXm/ygmV91A3sFFcJ+Yn/O9sfLwAWbT1HvjkvLdBWQFx2v0o?=
 =?us-ascii?Q?OEOLywDoLmsqj+nNzz2lPmFBXI1KMV71CGCWBMVFP2uE/F9/Nx/4tLa1jhYs?=
 =?us-ascii?Q?x/v+kz14udyNe31w0Zq2VTvi5ZvL25TyVjwNIKGGIPnnNzFdbfFVYkk6TCXE?=
 =?us-ascii?Q?TM8t5f1iOIqQWlnSxseYA1l7uVvgysS8yGcT47QYD4F+vL1li+rJeoyEFYnY?=
 =?us-ascii?Q?d13nLdJnZ7jLzf24XdbxPZDNrgCdGcHMymvpjwM9johvCJXCz9JRFxT4UCOK?=
 =?us-ascii?Q?qCx9N3VIvWo7DhmGPq73vF3SnJQN+84k?=
X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-MessageData-0:
 =?us-ascii?Q?78R/x1kyOu4jWT+bOH/KceuU2/P0TqAIQ1F8dcJ9jEKw/2U69vDrX11V6h/n?=
 =?us-ascii?Q?mscZ0SbrG1l3dsz/znVP0tqop1u1zH+PVZIpmInD8qO4afOpy4va5BIAFPyo?=
 =?us-ascii?Q?nsrzW1xvmtyZhnj6+WBPBB6goVxr0m0yUcEbir0+LvbVy8N5zNVsf2tQH8R2?=
 =?us-ascii?Q?W4Pvl6HDxD8wh8399q+poR7iQDon5ZISi+m2BdQq+qq3+Pzbf7sdhZfNIZSm?=
 =?us-ascii?Q?eWodgKa74wbC9dILL9FJDpJAjQpg+g9VO4rdAwLiFK9thucfW6IdGCgRuAI/?=
 =?us-ascii?Q?nAxfs+HgBwJaZGpDaUGtAwQLmMj4p5tUpkOb1acADIVX8ma/ZHk3hiVQFtGt?=
 =?us-ascii?Q?vbyEOhlUtBNqXAi8gSSMMzt4eeFMzWKG1RL08yxaENXyQUZ2wGhLfEfoKnxY?=
 =?us-ascii?Q?dvc6BC0YHkzFkOGhvrB/Sv1Ivp4BHAbjrWtfXbPFPFvVkzzffLnZrxg+x5UV?=
 =?us-ascii?Q?Ughe120IE+x70wNFGLeVwDKfoMPtq4Nu33/Zujj87BMKjykbC1P0qqKSt/OR?=
 =?us-ascii?Q?OE7yecUPXQ19R5mDFMH52ag6940TsmZ7VX6gMRMW9Vp/RKwhLIL2nMjeuA8X?=
 =?us-ascii?Q?+K5J4IIwz5hYMDX1g1pREA1DLpor0X4sm5A022sIZ47K+KDnHAMlPv5mvR41?=
 =?us-ascii?Q?Cy8AQP/hcR/pZ1HhqnABkyjmBaWq1/SXRSnty75XaINfrNdYFVKo6DlKnDvo?=
 =?us-ascii?Q?6fuEuV4NTPP9ru3FNzfjGhtjdL/VxjRWi5che4a0hB2u31yF6OUaDOWMdXMN?=
 =?us-ascii?Q?I3EeFgf74oxkRr5+U/FXK4+ueVu370dWklFL6MlkuIBTul0RuJ7iLuDzEVux?=
 =?us-ascii?Q?3e/Tfp462MJKgZpvq99Xs+MZ/IL+4qBeglCEOP/xGjmL8DCiTBnncAA7yO2+?=
 =?us-ascii?Q?/o+TbTYTKxFT2BiWDwQnFYaEYHzRtazgDcmiSTPTVjmpVyuuYCNq0KS+jVvl?=
 =?us-ascii?Q?fppKKLwxrLR22Ou56uE6Wn9DqaxOmF4jqD7yIEFg43OkbdNV9SjmZR6AYjvh?=
 =?us-ascii?Q?CJ6clSGb/3hQzhZ0QCWVUju2OouV3ZI7RFHJAvFNgRMCqiMErmd0ax7DXzAB?=
 =?us-ascii?Q?mG+S578eZ6VwBw1fqxlJnAmcOnnrfVJzUGm2+kxU2GdtJ5bg6i7YFOOT6o//?=
 =?us-ascii?Q?PZpAEh4bf5DZnUjV4PaGHdxQQQUr/XrZX8CeKslc5QlyClI9Br6NAlNVlnXC?=
 =?us-ascii?Q?sLM4/bSCcvXMsJmNrd7BQkb6UPlgX2v6oUPahuth6WMM19ohnfMjATiq1Mrt?=
 =?us-ascii?Q?6PHefJK4jWcYbKEMr8b0iwKdvpe1hUEbFCa6xd8wcq37UQyicEnq+T2oEgMW?=
 =?us-ascii?Q?y6/PyujCOH6ZD3PYVr2lCIGQlMkeOqBFHUL9VYOvj0TBgzRpNg5N6Dj/kKoF?=
 =?us-ascii?Q?Xw6WRKBUX3CoMmk+djKqvus60hdQpayoeHHQIdz7Ycww5c6OPILZ8y0IDXWp?=
 =?us-ascii?Q?nL4BK3/DVuuFlERunn7UsltBTvP3k9ij/8+XjXv4JSbvWawnx1PPbiwVPxVt?=
 =?us-ascii?Q?yYT77S9I7A5Ht1ZZGJCkkT/nwvukOwER1yQx?=

Answer 1

Can you use the instruction here, change the email to plain text or rich text format (basically avoid the html format) and see if it goes through:

https://support.microsoft.com/office/change-the-message-format-to-html-rich-text-format-or-plain-text-in-outlook-338a389d-11da-47fe-b693-cf41f792fefa