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

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: EXCH2016SERVER.SenderDomain.org
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 EXCH2016SERVER.SenderDomain.org (2002:c001:1dd::c001:1dd) by
EXCH2016SERVER.SenderDomain.org (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 EXCH2016SERVER.SenderDomain.org ([fe80::f1a1:c5d4:8fb0:2a25]) by
EXCH2016SERVER.SenderDomain.org ([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<Sender@SenderDomain.org>
To: “Recipient’s Email Address” <Recipient@RecipientDomain.edu>
Subject: email
Thread-Topic: email
Thread-Index: AddmvtJpSP5iLcn6RTW8XY2hs0zkhw==
Date: Mon, 21 Jun 2021 10:00:06 -0700
Message-ID: <5fec2334ee2a48de94b4648642829c33@SenderDomain.org>
Accept-Language: en-US
Content-Language: en-US
X-MS-TNEF-Correlator: <5fec2334ee2a48de94b4648642829c33@SenderDomain.org>
MIME-Version: 1.0
X-Originating-IP: []
Return-Path: Sender@SenderDomain.org


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8/22/2022 - Mon