XFOR: Preventing the Internet Mail Service From Relaying Unsolicited Commercial E-Mail Messages

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When you install the Internet Mail Service, it is configured by default to enable rerouting for Post Office Protocol version 3 (POP3) and Internet Message Access Protocol, Version 4rev1 (IMAP4) clients. This rerouting is found on the Routing tab of the Internet Mail Service object. The Internet Mail Service accepts and relays mail to non-local recipients. Message relay occurs when a client or remote SMTP server connects to the Internet Mail Service and submits messages for non-local recipients. If the Internet Mail Service does not restrict message relay, it may be used to relay UCE messages.

If your server configuration prevents the client from relaying mail, SMTP RCPT (receipt) commands that specify a non-local recipient are refused, and the following message is displayed: To configure relay restrictions in the registry, use the values in the following registry key: The following examples outline the value, the data type, and the function the value performs. After you make the changes, stop the Internet Mail Service, and then restart it.

• RelayFlags, REG_DWORD

  Defines which relay control rules are in effect.

• RelayDenyList, REG_MULTI_SZ

  Specifies hosts that cannot relay messages through your server.

• RelayAllowList, REG_MULTI_SZ

  Specifies hosts that can relay messages through your server.

• RelayLocalIPList, REG_MULTI_SZ

  Specifies the local IP addresses of the server that an SMTP client can connect to and relay mail. This is useful for multi-homed servers that have internal and external interfaces. Enabling IP forwarding disables this feature.

NOTE: RelayDenyList, RelayAllowList, and RelayLocalIPList consist of a net address and optional mask per line. Order is not important in these lists. Each line consists of two parts, the net address and the mask, separated by a semicolon. For example: If the mask is omitted, the default used is 255.255.255.255.

A net address matches a rule if the bitwise-AND of the IP address and the mask equals the net. That is: For example:

• To add net 192.168.0.0 to a list, add the following line to the list:

  192.168.0.0;255.255.0.0

• To add the host 192.168.1.17 to a list, add one of the following lines to the list:

  192.168.1.17;255.255.255.255
  
  - or -
  
  192.168.1.17

What follows is the logic that is used to determine if the client can relay mail. If none of these statements apply, the client is not allowed to relay mail.

• If bit 1 of RelayFlags is set (decimal value 1) and the IP address of the client matches a pattern in RelayDenyList, the client is not allowed to relay mail.
• If bit 2 of RelayFlags is set (decimal value 2) and the IP address of the client matches a pattern in RelayAllowList, the client is allowed to relay mail.
• If bit 3 of RelayFlags is set (decimal value 4) and the client is connected to a local IP address that matches a pattern in RelayLocalIPList, the client is allowed to relay mail.
• If bit 4 of RelayFlags is set (decimal value 8) and the client is authenticated, the client is allowed to relay mail.
• If only bit 1 is set, the client is allowed to relay mail.

For example:

• All clients not explicitly denied can relay mail. Set bit 1 of RelayFlags (set its decimal value to 1), and add a rule to RelayDenyList for each host or group of hosts that you want to deny. To prevent all hosts on the subnet 192.168.17.0 from relaying mail, add the following line to RelayDenyList:

  192.168.17.0;255.255.255.0

• All clients not explicitly allowed are denied. Set bit 2 of RelayFlags (set its decimal value to 2), and add a rule to RelayAllowLists for each host or group of hosts that you want to allow. To allow all hosts on subnet 192.168.1.0 to relay mail, add the following line to RelayAllowList.

  192.168.1.0;255.255.255.0

• Allow all hosts on a subnet except for a subset. To allow all hosts on a subnet, set bit 2 of RelayFlags (set its decimal value to 2), and add a rule to RelayAllowList to match the subnet. For the subnet 192.168.1.0, the following rule works:

  192.168.1.0;255.255.255.0

  To prevent a subset of the hosts on subnet 192.168.1.0 from relaying mail, also set bit 1 in RelayFlags in addition to bit 2, (which was set above); the net result is to set its decimal value to 3. Add the IP address of each host to RelayDenyList. If the subset of hosts is grouped together, you can add a single rule to match all of them. For example, if 192.168.1.1 through 192.168.1.7 are not allowed to relay, the following rule is adequate. Listing each address explicitly in RelayDenyList also works.

  192.168.1.0;255.255.255.248

• Allow clients that connect to the selected network interfaces to relay.
  
  This method is useful if the host has multiple network interfaces, and IP forwarding is not enabled. Set bit 3 of RelayFlags (set its decimal value to 4), and add the IP addresses of the network interfaces that relay mail to RelayLocalIPList.
• Allow authenticated clients to relay.
  
  Set bit 4 of RelayFlags (set its decimal value to 8) to allow clients that have authenticated (by using the Auth command) to relay mail.

After you create or modify these registry settings, in Control Panel, double-click Services, and then stop and restart the Internet Mail Service so that the changes take effect.

If you set the SMTP Interface Events diagnostics logging category to minimum or a higher logging level in the Internet Mail Service Diagnostic Logging property page, when a message is denied for relay through the Internet Mail Service an event is logged to the application event log. The event indicates the sender's IP address, sender's host name (if available), the sender's authentication account (if authentication was used), and the recipient address for the message.