When every any new Exchange 2007/2011 Hub role is installed, by default 2 receive connectors will be created and they are

 Default <Server name>: Works on port 587

Client <Server name>: Works on port 25

 These connectors are configured to receive messages from the Internet, from e-mail clients, and from other e-mail servers. These Hub servers are not configured for Authenticated relay. If your hub servers are configured to receive email from the Internet or from other email servers which cannot perform authentication then you may have to configure to allow Anonymous relay on the receive connectors

 Below cmdlet helps to configure email from any anonymous recipient

 Get-ReceiveConnector “Receive Connector Name” | Add-ADPermission -User “NT AUTHORITY\ANONYMOUS LOGON” -ExtendedRights “Ms-Exch-SMTP-Accept-Any-Recipient“