What would make one Echange 2007 recipient kick out NDR 5.1.0 and all other accounts can receive messages fine from same sender?I am getting complaints from a user telling me she isn't receiving emails from another company. I get them to send me an email, and I receive with no problem. My user can get emails from any other domain except this one. What can I do? I suspect I may need to check AD, but where?

Hi According to following article: Delivery status notifications in Exchange Server and in Small Business Server http://support.microsoft.com/kb/284204/en-us This code 5.1.0 indicates a general categorizer-based failure (bad address failure). An e-mail address or other attribute could not be found in the directory. This issue may occur if contact entries do not have the targetAddress attribute set. This issue occurs most frequently when MDAccess receives "object not found" errors from DSAccess when the Categorizer is doing the homeMDB lookup on a user. Therefore, please verify the recipient address is correct when the sender sends email to him. If the sender use Outlook, please do not use Autocomplete feature and manully type the recipient address. If the issue persists, please help me gather the NDR message for further check. Mike

Several users at that domain have tried to manually type the address and send it with the same result. This is what they get:Reporting-MTA: dns;sending.domain.org Final-Recipient: rfc822;user@mydomain.eduAction: failedStatus: 5.1.0X-Display-Name: User, Name