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:

• The domain name of the recipient is not listed in Exchange "accepted domains" list.

  Please check Add your own internet domain to the “Accepted Domains” list in our Exchange 2007 configuration guide. The same applies for Exchange 2010.

• The recipient email address is not assigned to any user

  Please check the email addresses and spelling!

• If the recipient is actually a distribution group and internal emails to that group work then it is a configuration problem

  Please use the Exchange Management Shell with the command Set-DistributionGroup GROUPNAME -RequireSenderAuthenticationEnabled $False to switch off the sender authenticaton requirement that gets switched on by default for new groups in Exchange 2007 and 2010

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".

 

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.