420 4.2.0 RESOLVER.ADR.Ambiguous; ambiguous address

We  are in the middle of a migration from Exchange 2003 to Exchange 2010.  I see there that there are over 100 items in the submission queue for Exchange 2010.  All of the items are from the same address (exchange_server_name-IS@domain.com) to the same recipient address (_5@domain.com).  All of the messages have the same error:

Identity: CAS01\Submission\257062
Subject: Folder Content
Internet Message ID: <012033945802AC4AB8DF73C9A4EB19639B9657@echange_server_name.domain.com>
From Address: echange_server_name-IS@domain.com
Status: Retry
Size (KB): 42
Message Source Name: SMTP:Default CAS01
Source IP: x.x.x.x
SCL: -1
Date Received: 4/25/2011 4:03:09 PM
Expiration Time: 4/27/2011 4:03:09 PM
Last Error: 420 4.2.0 RESOLVER.ADR.Ambiguous; ambiguous address
Queue ID: CAS01\Submission
Recipients:_5@domain.com

I went through the following links:

http://flaphead.dns2go.com/archive/2007/11/09/550-5-1-4-resolver-adr-ambiguous-ambiguous-address.aspx

https://www.experts-exchange.com/questions/23445951/NDR-550-5-1-4-in-Exchange-2007.html

When I do an LDAP search for the email recipient address (_5@domain.com) it doesn't come up in the search.  I confirmed my syntax was correct by finding an account that I know does exist.

I did find SMTP addresses on the System mailboxes in the storage groups of the Exchange 2003 server that had similar addresses like _2@domain.com  & _9@domain.com, so I'm wondering if the address _5@domain.com is a remnant.

Any help would be appreciated.