Closing an Open Mail Relay

Mail relaying, the forwarding of mail from one server to another, is a common and necessary component of SMTP (Internet mail) servers.  An open mail relay is found on SMTP servers that allow unidentified users make use of their relaying service.  This can be a problem because unscrupulous users can send a single message addressed to hundreds of user to the server with the open relay and the server will send out the messages for the user.  Mass mailers use open relays without permission to send hundreds of unsolicited e-mails to unsuspecting users.

Microsoft Exchange 5.5 will act as an open relay if it is not configured otherwise.  To close the open relay run the Exchange Administration program.  Open the configuration, connections container (see below) and double click on the Internet Mail Service.

When the Internet Mail Service (Mail) Properties Window opens, click on the Routing tab (see below).

When the Routing window opens, click on the Routing Restrictions button.  When the Routing Restrictions window appears check the Hosts and clients the successfully authenticate check box (see below).  You will be required to restart the Microsoft Exchange Internet Mail Service for this change to take effect.  After you have restarted the service or rebooted the server unauthorized users will be prevented from using your Exchange server to send mail.

If you are using your Exchange sever as a POP3 or IMAP4 server you may need to make changes on your Outlook Express or Outlook clients.   If that is the case, click on  Additional Changes for Outlook Express .