SPF fail using basic SMTP AUTH

Hello.

We just moved to Office 365 and almost everything is running just fine. But, when we are sending out mails from our ERP system, the mails somehow doesn't pass SPF and DKIM check, but also does pass 🤔

We are using SMTP with basic authentication.

Here is an example from a Gmail account that are blocking the message:

mx.google.com rejected your message to the following email addresses:

email address removed for privacy reasons
Your message wasn't delivered because the recipient's email provider rejected it.

mx.google.com gave this error:
Your email has been blocked because the sender is unauthenticated. Gmail requires all senders to authenticate with either SPF or DKIM. Authentication results: DKIM = did not pass SPF [xxxxxxx.com] with ip: [2a01:111:f400:fe1e::728] = did not pass For instructions on setting up authentication, go to https://support.google.com/mail/answer/81126#authentication 6a1803df08f44-6ae4b416ab0si85203426d6.346 - gsmtp

Diagnostic information for administrators:

Generating server: DU0PR09MB5907.eurprd09.prod.outlook.com

email address removed for privacy reasons
mx.google.com
Remote server returned '550-5.7.26 Your email has been blocked because the sender is unauthenticated. 550-5.7.26 Gmail requires all senders to authenticate with either SPF or DKIM. 550-5.7.26 Authentication results: 550-5.7.26 DKIM = did not pass 550-5.7.26 SPF [xxxxxx.com] with ip: [2a01:111:f400:fe1e::728] = did not pass 550-5.7.26 For instructions on setting up authentication, go to 550 5.7.26 https://support.google.com/mail/answer/81126#authentication 6a1803df08f44-6ae4b416ab0si85203426d6.346 - gsmtp'

Original message headers:

ARC-Seal: i=1; a=rsa-sha256; s=arcselector9901; d=microsoft.com; cv=none; b=biAtPmNZ/LaxEK6oNmV9c53LtHqgl3OQP3rWwtCevspx/mjw1EMBvmh4KzT+lGorrSagm95Nx9C5BXwWrv6V1BbXaZYZ8OMke/gFWXhk5fljxjnOvNtT9zDrO6sV+mnAl5mWfLHbnjV1uIXGWUG6Og7sNaOxcTo0zoc2n9qx2+qkRF6BLTq/kjHtYUQCi5APLKooF2fkCa0MJ+icOJRFqInUrSHE2Vc3xlPT/iWU1jOSg9eRDgVrlq7iBu4I+c1FdQOQlNj1OaLo1gIElVBl0BMCERvNkrVA+P/zJfgjT4V9z9voqmXgAB7hZ5MH10/LCI/pzY0GHsAdzucFOyqxfw==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=OsqzM2Cq/j4WaSWjXDk5WILYS6YmSmSAWJlIj56xkN4=; b=llDLukiByQk12bUAxRMho2pkkNVCRKCNJbABRFKwFbFXpQV5oKjocyIFnCFS/AeyjF2BDQiXcVbS4B+yTlbk6UhgO6s2XkuPWCApjyo9ziuUslM3880xKvUU0Qso77haF2oIg35ojAou6Kd9thmIajLjmcm4DrWssmC+E/Xrh5mbB4lqqzm+SSOHLpo6kB4jrfXI+gs57QrpZQ19wNpgfEakHIDHUhex+kcnZgKnoRPS7uD0UTgwdNrI/vP3jW1TJRFnzXTQs0lEmdsSfq92VRP0Ki89o1vWVOddMWTGW+NjoCPGPFlCUjTl+aLeR2D7TYI+2gVkPrpRSmGyo84m6g==ARC-Authentication-Results: i=1; mx.microsoft.com 1; spf=pass smtp.mailfrom=xxxxxx.com; dmarc=pass action=none header.from=xxxxxx.com; dkim=pass header.d=xxxxxx.com; arc=noneDKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=xxxxxx.com; s=selector1; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck; bh=OsqzM2Cq/j4WaSWjXDk5WILYS6YmSmSAWJlIj56xkN4=; b=NBFObzF6bN2RfA+WwC78rAePUCvDA2/YXeLC2wjhxi9lV4NkVDxW3slBwITag/EUsOoAn87yre9YaGoK2Gw4YIJZN/rJGh4ce8JAolGLIc1XZXr+DaAuYpNkIXPj5L26ICh2C6eBAPiEpdBUXOSDBZPiyC+A822DVlKZcmpXombbv2Y07Oo/IWafV+GunZRaRa+TmvHzGnHogDGBhD2ZJMNr44Rur2cgfluHQ6ybCgKuj2p0YlP30t98iEkRKfCwpx0bP+kXco3fcxIz85baomDw3HR/iiLcrAvGUWxLb34I+Ijxdnrn665VdB9d0bMZ4otkGql/GsYLCx/AnqkwkQ==Authentication-Results: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=xxxxxx.com;Received: from AM6PR09MB3399.eurprd09.prod.outlook.com (2603:10a6:20b:d9::17) by DU0PR09MB5907.eurprd09.prod.outlook.com (2603:10a6:10:3ea::6) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.7633.27; Mon, 3 Jun 2024 12:29:59 +0000Received: from AM6PR09MB3399.eurprd09.prod.outlook.com ([fe80::22bf:c280:4419:4133]) by AM6PR09MB3399.eurprd09.prod.outlook.com ([fe80::22bf:c280:4419:4133%4]) with mapi id 15.20.7633.021; Mon, 3 Jun 2024 12:29:59 +0000From: "xxxxxx" <email address removed for privacy reasons>To: email address removed for privacy reasonsDate: 3 Jun 2024 14:29:59 +0200Subject: SubjectContent-Type: multipart/mixed; boundary=--boundary_967_6af500b3-6092-49e7-8463-646cb3d6ac7cX-ClientProxiedBy: MM0P280CA0002.SWEP280.PROD.OUTLOOK.COM (2603:10a6:190:a::27) To AM6PR09MB3399.eurprd09.prod.outlook.com (2603:10a6:20b:d9::17)Return-Path: email address removed for privacy reasonsMessage-ID: <email address removed for privacy reasons>MIME-Version: 1.0X-MS-PublicTrafficType: EmailX-MS-TrafficTypeDiagnostic: AM6PR09MB3399:EE_|DU0PR09MB5907:EE_X-MS-Office365-Filtering-Correlation-Id: 90b95f77-3289-41ba-cd3e-08dc83c8e22eX-MS-Exchange-SenderADCheck: 1X-MS-Exchange-AntiSpam-Relay: 0X-Microsoft-Antispam:        BCL:0;ARA:13230031|366007|52116005|1800799015|376005|38350700005;X-Microsoft-Antispam-Message-Info:        =?us-ascii?Q?nA1m6WXTBdNqlaNQGutxzUfMvwkFiUR1mLHugzqZnkxIpvYNC/netSnW7MYb?= =?us-ascii?Q?E7L96OIo91gDO5935TPOKgwl+Qw3UNwjHwP0S9hNJ3VqSaUBiin0eD9UslGH?= 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When I login to the mailbox manually and send a mail to a Gmail it passes everything.

Does anyone know where to look?