Can't send email to my "Shared Mailbox" from outside my organization?

Question

I got a report from a department that "senders" from outside my organization can not send email to a shared mailbox. The outside user are receiving a bounce back 550 5.7.133 error message.

After logging into the Exchange Admin Center > Recipients > Mailboxes > <the name of the shared mailbox> >Manage mail flow settings > Message delivery restrictions.

I have it set to Accept messages from: All Senders.

Why all of a sudden am I having this issue? This Shared mailbox was created about two years ago.

Bounce back email message that the outside users are receiving:

Error Details
Reported error: 550 5.7.133 RESOLVER.RST.SenderNotAuthenticatedForGroup; authentication required; Delivery restriction check failed because the sender was not authenticated when sending to this group
DSN generated by: SA1PR18MB4534.namprd18.prod.outlook.com
Message Hops
HOP TIME (UTC) FROM TO WITH RELAY TIME
1 7/29/2021
6:22:10 PM [12.252.190.26] web-mail.mail.com *
2 7/29/2021
6:22:11 PM xxxx.xxx.com MW2NAM04FT054.mail.protection.outlook.com Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) 1 sec
3 7/29/2021
6:22:11 PM MW2NAM04FT054.eop-NAM04.prod.protection.outlook.com MW4PR03CA0319.outlook.office365.com Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) *
4 7/29/2021
6:22:11 PM MW4PR03CA0319.namprd03.prod.outlook.com SA1PR18MB4534.namprd18.prod.outlook.com Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) *
Original Message Headers

Received: from MW4PR03CA0319.namprd03.prod.outlook.com (2603:10b6:303:dd::24)
by SA1PR18MB4534.namprd18.prod.outlook.com (2603:10b6:806:1e1::8) with
Microsoft SMTP Server (version=TLS1_2,
cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4373.21; Thu, 29 Jul
2021 18:22:11 +0000
Received: from MW2NAM04FT054.eop-NAM04.prod.protection.outlook.com
(2603:10b6:303:dd:cafe::84) by MW4PR03CA0319.outlook.office365.com
(2603:10b6:303:dd::24) with Microsoft SMTP Server (version=TLS1_2,
cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4373.18 via Frontend
Transport; Thu, 29 Jul 2021 18:22:11 +0000
Authentication-Results: spf=pass (sender IP is xx.xxx.x.xxx)
smtp.mailfrom=email.com; our domain name here.com; dkim=pass (signature was verified)
header.d=mail.com;our domain name here.com; dmarc=bestguesspass action=none
header.from=email.com;compauth=pass reason=109
Received-SPF: Pass (protection.outlook.com: domain of email.com designates
xx.xxx.x.xxx as permitted sender) receiver=protection.outlook.com;
client-ip=xx.xxx.x.xxx; helo=xxxx.xxx.com;
Received: from xxxx.xxx.com (xx.xxx.x.xxx) by
MW2NAM04FT054.mail.protection.outlook.com (10.13.30.131) with Microsoft SMTP
Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id
15.20.4373.18 via Frontend Transport; Thu, 29 Jul 2021 18:22:11 +0000
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=mail.com;
s=dbd5af2cbaf7; t=1627582930;
bh=yPUfHsTrGQsmcAOfo5PuZbuUzZ1rNq4VK17uaAji9/0=;
h=X-UI-Sender-Class:From:To:Subject:Date;
b=W97DBo6HTLBimPvOvOKj22Wwxo/5p9Sl7PWc8NTxRYxrHyQoKjuMjPYwSHYbjTb8o
Pp7+grUkTGeWZo2XwWjHiGY6xBhLJ/LdQcYDPPdSJ71oxoESCpaW8M490QO682alNb
DIha/vCX2sJ/WCLP8KBw+Uhsta+dyuWteQ6BWjCI=
X-UI-Sender-Class: 214d933f-fd2f-45c7-a636-f5d79ae31a79
Received: from [xx.xxx.xxx.xx] ([xx.xxx.xxx.xx]) by web-mail.mail.com
(3c-app-mailcom-lxa11.server.lan [xx.xx.xx.xx]) (via HTTP); Thu, 29 Jul
2021 20:22:10 +0200
MIME-Version: 1.0
Message-ID: <trinity-8ed3245a-4e87-4d36-937f-cd8b7f0d4804-1627582930141@3c-app-mailcom-lxa11>
From: Ed Smith <pd187@tiedtlaw email .com>
To: utilitybilling@our domain name here.com
Subject: IT Department Test
Content-Type: text/html; charset=UTF-8
Date: Thu, 29 Jul 2021 20:22:10 +0200
Importance: normal
Sensitivity: Normal
X-Priority: 3
X-Provags-ID: V03:K1:oNsRPJ/j4TlHQPydlfeTWPl6G2Heun3VM9CkzogVrXemerZpAAKgv6jhW0wBCR4KfuGLK
TLFWI5jVnmqTmZLrE62NxPyboeDJVCfwf7TB5fnoOR9bDRt8sawWlXJLmN+9qLwoldsHQIG0XttB
kqX+O9+YnqWj2fNPpS7VcbwF/nmOEr1k3i6dQEVfLsJW9656UckdieQhLbHr6xPC8xKGG85JPpZE
PGYeMgSNq8oCY+X9g9NNU6tI8DphMBvBami0VS64xww5/bdNHzPqmwic1SYKRWHVI8to4bRcKSX7
bI=
X-Spam-Flag: NO
X-UI-Out-Filterresults: notjunk:1;V03:K0:LQeMWOtiAAQ=:7IX27hL4WG61O+LZwqJkct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Return-Path: pd187@tiedtlaw email .com
X-EOPAttributedMessage: 0
X-EOPTenantAttributedMessage: 2b701e36-32a0-441e-8824-1f7fde6fa173:0
X-MS-PublicTrafficType: Email
X-MS-Office365-Filtering-Correlation-Id: 59c4cd9d-d467-4894-5c0c-08d952bdc84b
X-MS-TrafficTypeDiagnostic: SA1PR18MB4534:
X-MS-Exchange-AtpMessageProperties: SA|SL
X-CLOUD-SEC-AV-Info: our domain name here,office365_emails, inline
X-MS-Oob-TLC-OOBClassifiers: OLM:1728;
X-Microsoft-Antispam: BCL:0;
X-Microsoft-Antispam-Message-Info:
=?us-ascii?Q?byUnSxGtO1cjHjSP558cS7j0Z0wbzeeoA2DvDAFKw5cql0+C0D+j/HFC0qKZ?=
=?us-ascii?Q?cep0IzUCWc6EJm2lbPBxwzQVI3gWAl2BDGnPprSI++1PaKSkfZjyPlgwsPfu?=
=?us-ascii?Q?mHISE4mWxcR0pJgCJuvncGk+7J9xajY0ZFz+HOePiJqBqmFujU+m0xGIYyfw?=
=?us-ascii?Q?2AzxMDHWvBcL1y36FBy7S9N3TtXXXNJ3YykyN0JBOHD0JcE/OQMN7FcIeUdc?=
=?us-ascii?Q?Sos2tThjXLxgCZlU9aQpFa0nw0QgDpPqozfQ5VcLZvyxJpVzywdFdlve8O80?=
=?us-ascii?Q?4Vbru0vB+47NvCclGunETN5w9AGmRH+WGmfXcmsyNQdGOz1cR6Lx+ux5xyH1?=
=?us-ascii?Q?PT0vNq6umYpCcTIyKypV+AeDCXp49jDNCzhtrtGwwSi9zi9TZh0H2Hbxe/Ky?=
=?us-ascii?Q?sC+cuLH/MTrtrk/JCLkWzK/+ruhSHaUoDmavmZsDSMtqwytUgdqBXtO7P3Bb?=
=?us-ascii?Q?uQcicd4YlC1EX93vM9bePj6NBNuZn9G7DQmYTvgwRce6MoSPimmAIJeaT63C?=
=?us-ascii?Q?s/ki8b7Bp7lUcpUVDBlz5TfothCHUY+/mMwRZmDvhCfxqNlxNwpBd1SizrZj?=
=?us-ascii?Q?OB27Z093NI5fSzSoH1oNAqKvY7qoIq72d+1i6Q5W2prGNHC/uZoH+l4zFlqD?=
=?us-ascii?Q?2opdAH+/h8GV6ZpcQK0U3XRLgl7Y1iJFeVaVg7AsK+N5kI5VsY0ZmltcAUlP?=
=?us-ascii?Q?porO1Uqz+IjaOqzpMyy5yDEDTvxl3Z8zqDsKKIL0IwIstHlxiSo/FVTBsgKg?=
=?us-ascii?Q?XWlY1eTiuszEn2Z9Gi7J5fMHeW6vLFQ9W27bYhEbm23OWCKrLPbjRjKFqkdA?=
=?us-ascii?Q?QGCMunDPze5w6XCBTR2U61UOG9QFhpdYRvj6f2/tZGA+k7wYAIk9taRDX4Mh?=
=?us-ascii?Q?l8KkNElI2UiRP3CD9jga6k4Vl3phCVY7QRh9CdZ0I7BHoCUEPDSlgtIuqcho?=
=?us-ascii?Q?vMgVd37+qm/1TVv2oCsk9bXiIvYlgfEa6Kz74aFkCWFqiklFZIqyXfBgcx4M?=
=?us-ascii?Q?bN5JCzS/vc/AoiGTyzoslL99vSe5ZWxaL9GtqExnP1BkGoSl0bREACUq3qoK?=
=?us-ascii?Q?y5BoWVLQ9U+OgzYEqCrDBkgNddZRdHexfndBlRgn58gpcRXBCMjWuMQU6tN/?=
=?us-ascii?Q?QtFUTs5LPcidQ/4FfhDHtdGW6K2QdHexC0b9Jl7il1ZGvVRRr8CCc9ql2+n2?=
=?us-ascii?Q?x8dSWm+CUP4x0J40LhXfsqG9EYlDXe/8qSJjGvkl6sdfcFeqiMdR0ySNBPkJ?=
=?us-ascii?Q?1kiwR86dQawPvgx7clTPp1FPCKvp0qf9KjRluxMcWB24fj/2hv5WQ6hVi7RF?=
=?us-ascii?Q?/Ccj9Naj4/ahrzxQwFXNX1pdOrfkb6GSsI+jgkenb+l0TAfU8Q2SibzP97q3?=
=?us-ascii?Q?cGf5DJBtjOYyuDEIxIFRbVdlKepVu400vPJbwwMOtTLXPFYqLtMJwtGA8yB3?=
=?us-ascii?Q?i52x8cKY81DXTvoFPtGKNM3wPo7IETpJ2U3yYZHzOASJtdttkUhT3ZtS0VD7?=
=?us-ascii?Q?5YrM3TPlRtcJdyjrxZ+BLSWgxnuAcl2mFoCF?=
X-Forefront-Antispam-Report:
CIP:74.208.4.201;CTRY:US;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:xxxx.xxx.com;PTR:xxxx.xxx.com;CAT:NONE;SFS:(4636009)(7916004)(8676002)(6916009)(956004)(336012)(26005)(5660300002)(356005)(7596003)(58800400005)(33716001)(9686003)(3480700007)(7116003)(1096003)(6706004)(558084003)(86362001)(239884005)(558944008);DIR:INB;
X-MS-Exchange-Transport-Forked: True

Answer 1

Hi @Keith Hampshire ,

Why all of a sudden am I having this issue? This Shared mailbox was created about two years ago.

Was any change made to this shared mailbox before the issue started?

From the error message "550 5.7.133 RESOLVER.RST.SenderNotAuthenticatedForGroup; authentication required....", seems like the issue is related to a group rather than a shared mailbox. So I am thinking if it's possible that you have accidently made any changes that would automaticaly forward message sent to the shared mailbox to another distribution group.

Please go to Exchange Admin Center > Recipients > Mailboxes > <the name of the shared mailbox> >Manage mail flow settings, choose Email forwarding and check if the shared mailbox has auto-forwarding enable for a group.

Also go through the mail flow rules to see if there's any rule set up that forwards messages sent to this shared mail to another group.

In case either of the above is true, check the Message delivery restrictions of the group involved, make sure it is set to allow all senders as well.

---

If an Answer is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.