<div>
What's the generating server? And why is messagelabs involved? <br />
<br />
In Exchange 2013, DSN 5.2.1 indicates issues with the recipient mailbox:<br />
<br />

<blockquote>
5.2.1 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<br />
Mailbox cannot be accessed &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<br />
The mailbox cannot be accessed. The mailbox may be offline, disabled, or the message has been quarantined by a rule. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<br />
Check to see if the recipient database is online, the recipient mailbox is disabled, or the message has been quarantined.<br />

</blockquote>
<br />
In other mail systems the cause might be different.
</div>


What's the generating server? And why is messagelabs involved?

In Exchange 2013, DSN 5.2.1 indicates issues with the recipient mailbox:



> 5.2.1      
> Mailbox cannot be accessed       
> The mailbox cannot be accessed. The mailbox may be offline, disabled, or the message has been quarantined by a rule.      
> Check to see if the recipient database is online, the recipient mailbox is disabled, or the message has been quarantined.


In other mail systems the cause might be different.

<div>
It's not compatible, you need to have exchange server 2010 sp3 to can migrate to office 365
</div>


It's not compatible, you need to have exchange server 2010 sp3 to can migrate to office 365

<div>
No other solutions were accurate. I fixed the problem myself
</div>


No other solutions were accurate. I fixed the problem myself

<div>
<div class="content wysiwyg-content">
Hi All,<br />
<br />
We are performing a staged migration from Exchange 2003 to Office365 and are experiencing the following error when users in the office365 environment send mail to users in the on premise exchange.<br />
<br />
Remote Server returned '&lt; #5.2.1&gt;' <br />
Original message headers:<br />
Received: from xxxx.xxxx.messagelabs.com ([xxx.xxx.xxx.xxx]) by &lt;FQDN &nbsp;on premise exchange&gt; with Microsoft SMTPSVC(6.0.3790.4675);<br />
<br />
Any ideas?<br />
<br />
Thanks
</div>
</div>


Hi All,

We are performing a staged migration from Exchange 2003 to Office365 and are experiencing the following error when users in the office365 environment send mail to users in the on premise exchange.

Remote Server returned '< #5.2.1>' 
Original message headers:
Received: from xxxx.xxxx.messagelabs.com ([xxx.xxx.xxx.xxx]) by <FQDN  on premise exchange> with Microsoft SMTPSVC(6.0.3790.4675);

Any ideas?

Thanks

Office 365 to Exchange 2003 (staged migration) NDR 5.2.1 error sending mail

Office 365 is a group of software plus services subscriptions that provides productivity software and related services to its subscribers. Office 365 allows the use of Microsoft Office apps on Windows and OS X, provides storage space on Microsoft's cloud storage service OneDrive, and grants 60 Skype minutes per month. Office 365 includes e-mail and social networking services through hosted versions of Exchange Server, Skype for Business Server, SharePoint and Office Online, integration with Yammer, as well as access to the Office software. All of Office 365's components can be managed and configured through an online portal; users can be added manually, imported from a CSV file, or Office 365 can be set up for single sign-on with a local Active Directory using Active Directory Federation Services.

Microsoft 365

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.