When a Microsoft Exchange server version 2010 is used as a downstream server where the SMG is configured with the following configuration, the sender may receive an non-deliry report (NDR, also known as the 'undeliverable e-mail') back after sending an e-mail to a distribution list that contains only a single user, where the sender is outside of the organization and is not a part of the Active Directory domain that the Exchange server is a part of. These NDRs have a copy of the message as well as the error with the error code of "550" mentioned below.
Error 550 5.7.1 RESOLVER.RST .AuthRequired; authentication required
The "Preserve recipient addresses" setting needs to be disabled by unchecking that configuration option and saving the change. This will allow for e-mails to be delivered successfully.
Applies To
Configuration that will trigger the above error: