Outlook Express wireless setting

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Closing relays

In order not to be considered "open," an e-mail relay should be configured to accept and forward only the following messages (details will vary from system to system - in particular, further restrictions may well apply):[14]

• Messages from local IP addresses to local mailboxes
• Messages from local IP addresses to non-local mailboxes
• Messages from non-local IP addresses to local mailboxes
• Messages from clients that are authenticated and authorized

In particular, a properly secured SMTP mail relay should not accept and forward arbitrary e-mails
from non-local IP addresses to non-local mailboxes by an unauthenticated or unauthorized user.

In general, any other rules which an administrator chooses to enforce (for instance, based on what an e-mail gives as its own envelope from address) must be in addition to, rather than instead of, the above.[14] If not, the relay is still effectively open (for instance, by the above rules): it is easy to forge e-mail header and envelope information, it is considerably harder to successfully forge an IP address in a TCP/IP transaction due to the three-way handshake that occurs as a connection is started.

Open relays have also resulted from security flaws in software, rather than misconfiguration by system administrators.[15][16] In these cases, security patches need to be applied to close the relay.
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