This code may occur if the 'legacyExchangeDN' attribute on the recipient object is blank or contains an invalid value. The ...

{4}: This code may occur if the 'legacyExchangeDN' attribute on the recipient object is blank or contains an invalid value. The Recipient Update Service may be having a problem or a third-party address generating tool may be failing to populate the attribute. Troubleshooting: Verify that the 'legacyExchangeDN' attribute of the recipient having this issue contains the correct value.