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

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 ([]) by <FQDN  on premise exchange> with Microsoft SMTPSVC(6.0.3790.4675);

Any ideas?

Who is Participating?
lutherpendragonAuthor Commented:
I've found the solution. That NDR can also be linked to one or more errors with ID 9667 generated in the Application event log on the Exchange server.

The underlying error ( ID 9667 ) mentions that the limit of Named Properties has been reached for the database.

The WORKAROUND (not log term fix) is documented here for exchange 2003:

Involves editing the non MAPI registry entry, As stated this is a workaround and not a long term solution.
Vasil Michev (MVP)Commented:
What's the generating server? And why is messagelabs involved?

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

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.
Mai Ali (MVP)Senior Infrastructure ConsultantCommented:
It's not compatible, you need to have exchange server 2010 sp3 to can migrate to office 365
lutherpendragonAuthor Commented:
No other solutions were accurate. I fixed the problem myself
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