Emails going to junk only on personalized outlook email address

[tsympil]

Hi,

I am using the MailGun API, I've begun sending regular emails to users of my web service. All was well until a user with an personalized outlook email (of my company).
Email was received with SCL 5, but emails to the regular @outlook.com address were received with SCL 1. Also emails were successfully received on gmail, etc.

Maybe the email headers will help determine why this is happening:

Received: from LO2P265MB5496.GBRP265.PROD.OUTLOOK.COM (2603:10a6:600:25f::6)
 by LO4P265MB5961.GBRP265.PROD.OUTLOOK.COM with HTTPS; Wed, 19 Oct 2022
 10:32:49 +0000
Received: from LO4P123CA0024.GBRP123.PROD.OUTLOOK.COM (2603:10a6:600:151::11)
 by LO2P265MB5496.GBRP265.PROD.OUTLOOK.COM (2603:10a6:600:25f::6) with
 Microsoft SMTP Server (version=TLS1_2,
 cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.5723.33; Wed, 19 Oct
 2022 10:32:48 +0000
Received: from LO2GBR01FT023.eop-gbr01.prod.protection.outlook.com
 (2603:10a6:600:151:cafe::d9) by LO4P123CA0024.outlook.office365.com
 (2603:10a6:600:151::11) with Microsoft SMTP Server (version=TLS1_2,
 cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.5723.34 via Frontend
 Transport; Wed, 19 Oct 2022 10:32:48 +0000
Authentication-Results: spf=pass (sender IP is <SENDER_IP>)
 smtp.mailfrom=<SERVICE_DOMAIN>; dkim=pass (signature was verified)
 header.d=<SERVICE_DOMAIN>;dmarc=bestguesspass action=none
 header.from=<SERVICE_DOMAIN>;compauth=pass reason=109
Received-SPF: Pass (protection.outlook.com: domain of <SERVICE_DOMAIN> designates
 <SENDER_IP> as permitted sender) receiver=protection.outlook.com;
 client-ip=<SENDER_IP>; helo=m204-245.eu.mailgun.net; pr=C
Received: from m204-245.eu.mailgun.net (<SENDER_IP>) by
 LO2GBR01FT023.mail.protection.outlook.com (10.152.42.149) with Microsoft SMTP
 Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id
 15.20.5746.16 via Frontend Transport; Wed, 19 Oct 2022 10:32:46 +0000
DKIM-Signature: a=rsa-sha256; v=1; c=relaxed/relaxed; d=<SERVICE_DOMAIN>; q=dns/txt;
 s=mta; t=1666175565; x=1666261965; h=Content-Type:
 Content-Transfer-Encoding: Message-Id: To: To: From: From: Subject:
 Subject: Mime-Version: Date: Sender: Sender;
 bh=rYRW1ou1ftL0yonAvATye33QWTb1fX13zGOCfYQKFgE=; b=nduw3OCCpptg3yAK46Zmynif56vc8akN3QmRdKX6/XHJD4WlAoTm1876u6XJOMY3BV34xBfR
 v+PSugNO6o91dSKTl+b/3P4fluYgxpY8FnzFGmxu1Sum+UI48es18FVR8UwmNsImCZT5TgRb
 8w0GZtpjbH8guVrli66fqD3OC6I=
X-Mailgun-Sending-Ip: <SENDER_IP>
X-Mailgun-Sid: WyI0YjNmMCIsInl1cmkudm9sa292aWNoQHplYnJhY2MuY28udWsiLCJiNTgxYTEiXQ==
Received: from <unknown> (<unknown> []) by
 api-n05.prod.eu-central-1.postgun.com with HTTP id
 634fd24c9ae17f1c165a12c7; Wed, 19 Oct 2022 10:32:44 GMT
Sender: info@<SERVICE_DOMAIN>
Date: Wed, 19 Oct 2022 10:32:44 +0000
Mime-Version: 1.0
Subject: Email subject
From: <SERVICE_DOMAIN> <info@<SERVICE_DOMAIN>>
To: <RECIPIENT>@<PERSONALIZED_EMAIL_ADDRESS>
Message-Id: <20221019103244.95151df169f1c4e7@<SERVICE_DOMAIN>>
Content-Transfer-Encoding: 7bit
Content-Type: text/html; charset=ascii
Return-Path: bounce+75884d.b581a1-<RECIPIENT>=<PERSONALIZED_EMAIL_ADDRESS>@<SERVICE_DOMAIN>
X-MS-Exchange-Organization-ExpirationStartTime: 19 Oct 2022 10:32:46.4741
 (UTC)
X-MS-Exchange-Organization-ExpirationStartTimeReason: OriginalSubmit
X-MS-Exchange-Organization-ExpirationInterval: 1:00:00:00.0000000
X-MS-Exchange-Organization-ExpirationIntervalReason: OriginalSubmit
X-MS-Exchange-Organization-Network-Message-Id:
 63890111-e012-49c8-b4a4-08dab1bd4370
X-EOPAttributedMessage: 0
X-EOPTenantAttributedMessage: 1f22ea5d-169a-4ced-baa6-277bdbf23917:0
X-MS-Exchange-Organization-MessageDirectionality: Incoming
X-MS-PublicTrafficType: Email
X-MS-TrafficTypeDiagnostic: LO2GBR01FT023:EE_|LO2P265MB5496:EE_
X-MS-Exchange-Organization-AuthSource:
 LO2GBR01FT023.eop-gbr01.prod.protection.outlook.com
X-MS-Exchange-Organization-AuthAs: Anonymous
X-MS-Office365-Filtering-Correlation-Id: 63890111-e012-49c8-b4a4-08dab1bd4370
X-MS-Exchange-Organization-SCL: 5
X-Forefront-Antispam-Report:
 CIP:<SENDER_IP>;CTRY:US;LANG:en;SCL:5;SRV:;IPV:NLI;SFV:SPM;H:m204-245.eu.mailgun.net;PTR:m204-245.eu.mailgun.net;CAT:SPM;SFS:(13230022)(451199015)(1690799008)(166002)(356005)(7596003)(7636003)(7116003)(8676002)(26005)(558084003)(1096003)(6916009)(22186003)(58800400005)(5660300002)(7846003)(3480700007)(2616005)(83380400001)(15650500001)(336012)(1076003)(46492015)(84240200001);DIR:INB;
X-Microsoft-Antispam: BCL:0;
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 19 Oct 2022 10:32:46.3647
 (UTC)

X-MS-Exchange-CrossTenant-Network-Message-Id: 63890111-e012-49c8-b4a4-08dab1bd4370
X-MS-Exchange-CrossTenant-Id: 1f22ea5d-169a-4ced-baa6-277bdbf23917
X-MS-Exchange-CrossTenant-AuthSource:
 LO2GBR01FT023.eop-gbr01.prod.protection.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Anonymous
X-MS-Exchange-CrossTenant-FromEntityHeader: Internet
X-MS-Exchange-Transport-CrossTenantHeadersStamped: LO2P265MB5496
X-MS-Exchange-Transport-EndToEndLatency: 00:00:03.0141087
X-MS-Exchange-Processed-By-BccFoldering: 15.20.5723.033
X-Microsoft-Antispam-Mailbox-Delivery:
  wl:1;pcwl:1;ucf:0;jmr:0;auth:0;dest:I;ENG:(910001)(944506478)(944626604)(920097)(811239)(250001)(410001)(930097);
X-Microsoft-Antispam-Message-Info:
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