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What can I do when exchange does not accept some emails and comes up with an error message: "550.5.5.1 user unknown" or "503.5.5.2 need Rcpt command" ?

Exchange 2007 and higher:

These are the most probable causes:

Exchange 2003:

The most common cause is recipient filtering in Exchange, but please check also sender, connection or intelligent filtering.

Check these configuration pages in the Exchange System Manager:

On the "Recipient Filtering" page the option should be turned off (ensure that "Filter recipients..." is not activated):

 

Ensure also that the "Drop connection if address matches filter" option is turned off on the "Sender Filtering" page:

 

The connection filter configuration should be empty:

 

On the "Intelligent Message Filtering" tab, in the Gateway Blocking Configuration please ensure that "When blocking messages:" is set to "No Action" or anything else but not "Reject".

Intelligent Message Filtering tab

 

09-11-2005: Just found out that some antivirus products, notably "Symantec Mail Security" also have a built-in antispam feature that causes the same error message.