Today we started having this problem with a external domain that has been working fine before.  This is not happening to all emails or all domains.  

Last week there was problem with a co.uk domain and that went away by itself (i know...)

I've done the tests with Mxtoolbox and exchange connectivity and they all pass. 

The Exchange 2013 server forwards to Smarthost (GFI) 

Server Exchange 2013

Clients: outlook 2010/2013

Diagnostic information for administrators:
Generating server: myserver.mydomain.com
user@externaldomain.com
Remote Server returned '< #5.5.0 smtp;550-Please turn on SMTP Authentication in your mail client. >'
Original message headers:
Received: from myserver.mydomain.com ([10.0.0.16] RDNS failed) by myserver.mydomain.com
 with Microsoft SMTPSVC(7.5.7601.17514); Wed, 17 Jun 2015 11:42:42 -0700
Received: from EMSRV.WL.local (10.0.0.16) by EMSRV.WL.local (10.0.0.16) with Microsoft SMTP
 Server (TLS) id 15.0.1044.25; Wed, 17 Jun 2015 11:38:34 -0700
Received: from EMSRV.WL.local ([fe80::4096:a0c:3cef:13c0]) by EMSRV.WL.local ([fe80::4096:a0c:3cef:13c0%15])
 with mapi id 15.00.1044.021; Wed, 17 Jun 2015 11:38:34 -0700
From: myUser
To: External user
Subject: K
Thread-Topic: K
Thread-Index: AdCpLMPIwf2IRxL0TEmNJMzqXdSUGg==
Date: Wed, 17 Jun 2015 18:38:34 +0000
Message-ID: <459b7995e3d243ad93a61a0f1da9ff73@EMSRV.WL.local>
Accept-Language: en-US
Content-Language: en-US
X-MS-Has-Attach: yes
X-MS-TNEF-Correlator:
x-ms-exchange-transport-fromentityheader: Hosted
x-originating-ip: [10.0.0.48]
Content-Type: multipart/related;
boundary="_007_459b7995e3d243ad93a61a0f1da9ff73EMSRVWLlocal_";
type="multipart/alternative"
MIME-Version: 1.0
Return-Path: MyUser@mydomain.com
X-OriginalArrivalTime: 17 Jun 2015 18:42:42.0971 (UTC) FILETIME=[652B66B0:01D0A92D]

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• Edited by Sunny Sra 11 hours 59 minutes ago

Hi Sunny,

Thank you for your question.

The error was not caused by the Exchange server, you could contact the network administrator to check network issue, especially in route which is facing Internet. There are some similar cases for us to refer by the following links:

https://social.technet.microsoft.com/forums/exchange/en-US/932093f9-cca5-40a5-a659-5d69c8d23c33/mail-rejected-because-of-550-enable-authentication-error

https://social.technet.microsoft.com/Forums/en-US/0d6f2e3b-7156-4f6b-9fe1-0401b7a2d3e7/please-turn-on-smtp-authentication-in-your-mail-client-in-ecxhange?forum=exchangesvrdeploylegacy

If there are any questions regarding this issue, please be free to let me know.

Best Regard,

Jim