Sometimes if you configure an external users to use the smtp service  with authentication (Relay Exchange)  of your exchange 2010 server, you can experience this error “Unable to Relay”:


Smtp connector properties Relay Exchange

“550 5.7.1 Unable to relay”

this is becose the default smtp connector does not have basic authentication enabled by default

First, on your default smtp connector, you have to uncheck this option:

  • Basic Authentication
    • Offer Basic authentication only after starting TLS

Second, assing rights to the users to send mails to any email address:

Get-ReceiveConnector “Default SMTP” | Add-ADPermission -User “DOMAINUsers” -ExtendedRights “ms-Exch-SMTP-Accept-Any-Recipient”

Attention! if you include “AUTHORITYANONYMOUS LOGON” as user then you can have a open Relay!!:

do not do this:

Get-ReceiveConnector “Default SMTP”|Add-ADPermission -User “NT AUTHORITYANONYMOUS LOGON” -ExtendedRights “ms-Exch-SMTP-Accept-Any-Recipient”


my latest development (Multiplayer Online Charts game):