User receives NDR when sending to a particular external email recipient.

Rudolf Amarlapudi 431 Reputation points


We are running an Exchange 2016 server (Version 15.1 ‎(Build 2176.2)‎).

One user is experiencing difficulties sending email to an external user. I found that email to this recipient was successful once I deleted the entry from Auto-Fill, and entered it manually. However, when it becomes available again, it keeps bouncing back with the following error further below.

I've also taken the following action:

  1. Deleted Auto-Fill entry.
  2. Deleted the Contact from the Sender's Contact folder.
  3. Created new Outlook profile.

I'm not sure where the point of failure is.

Any suggestions would be greatly welcome.

Thanks in advance.


Delivery has failed to these recipients or groups:
“Recipient’s Email Address”
The email address you entered couldn't be found. Please check the recipient's email address and try to resend the message. If the problem continues, please contact your email admin.
Diagnostic information for administrators:
Generating server:
Remote Server returned '550 5.1.11 RESOLVER.ADR.ExRecipNotFound; Recipient not found by Exchange Legacy encapsulated email address lookup'
Original message headers:
Received: from (2002:c001:1dd::c001:1dd) by (2002:c001:1dd::c001:1dd) with Microsoft SMTP Server
(version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384_P384) id
15.1.2176.14; Mon, 21 Jun 2021 10:00:06 -0700
Received: from ([fe80::f1a1:c5d4:8fb0:2a25]) by ([fe80::f1a1:c5d4:8fb0:2a25%12]) with mapi id
15.01.2176.014; Mon, 21 Jun 2021 10:00:06 -0700
Content-Type: application/ms-tnef; name="winmail.dat"
Content-Transfer-Encoding: binary
From: Sender Name<>
To: “Recipient’s Email Address” <>
Subject: email
Thread-Topic: email
Thread-Index: AddmvtJpSP5iLcn6RTW8XY2hs0zkhw==
Date: Mon, 21 Jun 2021 10:00:06 -0700
Message-ID: <>
Accept-Language: en-US
Content-Language: en-US
X-MS-TNEF-Correlator: <>
MIME-Version: 1.0
X-Originating-IP: []

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Accepted answer
  1. JeffYang-MSFT 6,231 Reputation points

    Hi @Rudolf Amarlapudi ,

    Do other users in the same organization have the same issue when using Auto-Fill entry to send emails to the same external user?
    To confirm if the issue is related to the Outlook desktop client side, it is suggested for you to try the same operations on your web mail and see if the issue has any difference.

    First of all, I would agree with the suggestions michev mentioned above, starting a new thread instead of replying to old emails and avoiding using the auto-complete entry of this user as much as possible should be able to help prevent this issue.

    By the way, based on my experience, there is also some cached recipient information which is non-visible and may cause similar issues as well. We could try to delete them using the MFCMAPI tool:

    1. Download and extract the MFCMAPI tool, launch MFCMAPI.exe, click OK, on the Session Menu, click Logon, select the Outlook profile for the mailbox, click OK.
    2. Expand Root Container - Top of Information Store - Contacts - Recipient Cache, right click on it, choose Delete folder, click OK:
    3. Exit the MFCMAPI tool and restart your Outlook to test.

    Hope these could help.

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1 additional answer

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  1. Vasil Michev 85,001 Reputation points MVP

    Is he replying to older emails? Basically ask him to start a new thread, and type in the address without selecting any auto-complete or contact entry.

    1 person found this answer helpful.
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