550 5.1.1 User unknown

Hello, I'm trying to send a message from outside to a known e-mail address  but am coming across a 550 5.1.1 ... User unknown error message.
Emails to another mailboxes of the same domain receiving without any issues. AD user of this mailbox is not locked and disabled.Mailbox is not full. Receiving emails from internal users working. Please assist
twolinesAsked:
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Alan HardistyCo-OwnerCommented:
Well - the obvious answer is that the mailbox doesn't exist - but it may be that the mailbox server is clustered and one of the clusters is missing the mailbox and needs updating.

Are you sure the address is still valid?
twolinesAuthor Commented:
Yes, the address is valid, I wrote above that this mailbox can receive emails from internal users but from outside - no. Other mailboxes of this domain can receive emails from outside, but this mailbox - no.
awaggonerCommented:
Are you using a spam/content filter?  Some of the cloud based services allow you to configure it so only valid email addresses can receive mail.  The problem is that you must maintain your addresses with the service.  Is this a new mailbox or an existing one.

Are you able to send email from the address to your outside account?  After receiving a test email from the internal account to the external one, do a reply back to the sender.

Can you find the messages in Exchanges Message Tracker?
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twolinesAuthor Commented:
There is no spam/content filter.  This is not new mailbox, this is existing one. In the beginning of March this mailbox was moved from one mailbox database to another. I can receive emails from this mailbox, but when I click reply and send back, still receiving 550 5.1.1 User unknown
Alan HardistyCo-OwnerCommented:
Do you have access to both servers?
twolinesAuthor Commented:
Yep. Provider has Exchange 2007 (mailbox in question) and Exch2010  and answer User unknown comes from HUB of 2010
Alan HardistyCo-OwnerCommented:
Has the Exch 2007 GAL been updated since the mailbox move?
twolinesAuthor Commented:
Yes
Alan HardistyCo-OwnerCommented:
And is the default Email address showing on the 2007 server correct?
twolinesAuthor Commented:
yes, email address is correct
Alan HardistyCo-OwnerCommented:
Can the mailbox be moved back to where it came from?
twolinesAuthor Commented:
Nope
Alan HardistyCo-OwnerCommented:
Can you please post the headers of an email from the problem user and obscure the email address / IP Addresses / Domain Names etc.

Thanks

Alan

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