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Double check your sent connectors, make sure you have created your own connectors and disabled or removed default one. Make sure you're using your MX record in send connector and good idea is to implement DKIM in your DNS<br />
<br />
Also read this - it can help<br />
<br />
<a href="http://www.shudnow.net/2013/06/04/how-anonymous-relay-works-in-exchange-2013/" rel="ugc">http://www.shudnow.net/2013/06/04/how-anonymous-relay-works-in-exchange-2013/</a>
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Cian Dev

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Exchange 2013<br />
Outlook 2013/2016<br />
<br />
550 5.7.1 Unable to relay on mails to certain recipients <br />
<br />
Is there anything i need to look at on exchange server<br />
<br />
Microsoft remote connectivity analyzer, all test passed OK<br />
<br />
Thanks in advance<br />
<br />
Cian
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Exchange 2013
Outlook 2013/2016

550 5.7.1 Unable to relay on mails to certain recipients 

Is there anything i need to look at on exchange server

Microsoft remote connectivity analyzer, all test passed OK

Thanks in advance

Cian

550 5.7.1 Unable to relay Exchange 2013

Exchange is the server side of a collaborative application product that is part of the Microsoft Server infrastructure. Exchange's major features include email, calendaring, contacts and tasks, support for mobile and web-based access to information, and support for data storage.

Exchange

Microsoft Outlook is a personal information manager from Microsoft, available as a part of the Microsoft Office suite. Although often used mainly as an email application, it also includes a calendar, task manager, contact manager, note-taker, journal, and web browser.