Mail flow issue from office 365 to Exchange 2003 On premises

Hi , We have Exchange 2003 environment & office 365 for exchange online. Slowy slowly we are moving the users from Exchange 2003 > 2010 > then cloud. We are experience an issue when office 365 user send a mail to exchange 2003 on premises user, for a particular db ( hosted on exchange 2003) , office 365 user getting a ndr message & saying that  
"The message reached the recipient email system but delivery was refused. Attempt to resend the message #5.2.1"

When I checked the Server, I found two event Id is generated, 9667 & 1025.

Any one facing the same issue, please suggest.
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hi padas6,

The SMTP error code 5.2.1 normally signifies:

the message size might be bigger than is allowed by the settings on either the mail store DB or the individual user mailbox settings.
that the mailbox is disabled
that the mailbox DB is offline

it might also be quarantined by a mail content filter rule.

padas6Author Commented:
Hi Wynand,

But in this case none the above is applicabel.

we are getting Event ID : 9667 which has described below link

We have implemented the things but only dismount & mount the database.

Waiting for Restart the Server.
padas6Author Commented:
there is some mailbox corruption of the couple of user, not sure, but after moving those mailboxes to another users , mailflow between on premises & cloud ( for those users ) are going smoothly.

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padas6Author Commented:
issue has been resolved by moving the mailboxes to another Server & database.
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