Office 365 Messages Undeliverable to known existing good address

Occasional Visitor

I have a 365 client that is trying to E-mail to known good address but gets a bounce back that the address can't be found.  I can E-mail the same address from any other source and it goes through.  But from the 365 users, it won't go.   Below is the bounce we are receiving.

I'd like to call 365 support but can't find a number.  Does anyone have the number?



Your message to couldn't be delivered.
chris wasn't found at
azieglerOffice 365chris
Action Required Recipient
Unknown To address  

How to Fix It
The address may be misspelled or may not exist. Try one or more of the following:
  • Send the message again following these steps: In Outlook, open this non-delivery report (NDR) and choose Send Again from the Report ribbon. In Outlook on the web, select this NDR, then select the link "To send this message again, click here." Then delete and retype the entire recipient address. If prompted with an Auto-Complete List suggestion don't select it. After typing the complete address, click Send.
  • Contact the recipient (by phone, for example) to check that the address exists and is correct.
  • The recipient may have set up email forwarding to an incorrect address. Ask them to check that any forwarding they've set up is working correctly.
  • Clear the recipient Auto-Complete List in Outlook or Outlook on the web by following the steps in this article: Fix email delivery issues for error code 5.1.10 in Office 365, and then send the message again. Retype the entire recipient address before selecting Send.
If the problem continues, forward this message to your email admin. If you're an email admin, refer to the More Info for Email Adminssection below.

Was this helpful? Send feedback to Microsoft.

More Info for Email Admins
Status code: 550 5.1.10 

This error occurs because the sender sent a message to an email address hosted by Office 365 but the address is incorrect or doesn't exist at the destination domain. The error is reported by the recipient domain's email server, but most often it must be fixed by the person who sent the message. If the steps in the How to Fix It section above don't fix the problem, and you're the email admin for the recipient, try one or more of the following:

The email address exists and is correct - Confirm that the recipient address exists, is correct, and is accepting messages.

Synchronize your directories - If you have a hybrid environment and are using directory synchronization make sure the recipient's email address is synced correctly in both Office 365 and in your on-premises directory.

Errant forwarding rule - Check for forwarding rules that aren't behaving as expected. Forwarding can be set up by an admin via mail flow rules or mailbox forwarding address settings, or by the recipient via the Inbox Rules feature.

Recipient has a valid license - Make sure the recipient has an Office 365 license assigned to them. The recipient's email admin can use the Office 365 admin center to assign a license (Users > Active Users > select the recipient > Assigned License > Edit).

Mail flow settings and MX records are not correct - Misconfigured mail flow or MX record settings can cause this error. Check your Office 365 mail flow settings to make sure your domain and any mail flow connectors are set up correctly. Also, work with your domain registrar to make sure the MX records for your domain are configured correctly.

For more information and additional tips to fix this issue, see Fix email delivery issues for error code 5.1.10 in Office 365.

Original Message Details
Created Date:6/7/2018 6:55:21 PM

Error Details
Reported error:550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient not found by SMTP address lookup
DSN generated

Message Hops
6:55:21 PM*
6:55:21 PM SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384)*

Original Message Headers

Authentication-Results:; dkim=none (message not signed)
 header.d=none;; dmarc=none action=none;
Received: from ( by ( with Microsoft SMTP
 Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id
 15.20.841.12; Thu, 7 Jun 2018 18:55:21 +0000
Received: from
 ([fe80::b04a:7b73:b351:5b3e]) by
 ([fe80::b04a:7b73:b351:5b3e%4]) with mapi id 15.20.0841.011; Thu, 7 Jun 2018
 18:55:21 +0000
Content-Type: application/ms-tnef; name="winmail.dat"
Content-Transfer-Encoding: binary
From: Alan Ziegler <>
To: Chris Goodwin <>
Subject: Test
Thread-Topic: Test
Thread-Index: AQHT/pEVVXKW1fZGlEKK+n9IxtJiRw==
Date: Thu, 7 Jun 2018 18:55:21 +0000
Message-ID: <>
Accept-Language: en-US
Content-Language: en-US
X-MS-TNEF-Correlator: <>
MIME-Version: 1.0
X-Originating-IP: []
X-MS-PublicTrafficType: Email
X-Microsoft-Exchange-Diagnostics: 1;BYAPR05MB4438;35:BpRwxBZfikmUK2afwVoJEKaBUwYbuDjYlYJeURdKfybEC56m6L8vXq0RbEn7oDYK9o7z22nwVlWVTv6baOtc/A==;38:48o+hC+afquzNwQTBL3NsJ+PVUszZOz/sZGi8qeG0N9M2mlffA69hDSZR+yJJMVx868rexk+WjaLO3MWFM61Vlhfwx0PuKaRs4mQSHPjB7bNRHfQPfYmDfZQQGvC4b4X4Ehklhtoqi9GtAOJt+44cuXDrK8ei+2VLoKSqz6veoU9Y3KDFvuYmmsHLdlpOUOlsgDv4YZWcIWqVqmVb1lRqyCQ6k/UCFMQU/KIJzmF4Y7Gql/adoUa/4WmW/XOi7ex
X-MS-Exchange-Antispam-SRFA-Diagnostics: SOS;
X-MS-TrafficTypeDiagnostic: BYAPR05MB4328:
1 Reply

@Brian Reichert Similar issue with one of our shared mailboxes. But there are no emails delivered from any domain. Did you ever figure out a solution? -O'Fetfa