Automatically route SMTP on X.400 over X.25

Many people who use SMTP and other messaging protocols based on the Internet Engineering Task Force (IETF) Request for Comments (RFC) 822 want to communicate with users on X.400-based systems. To make this communication possible, Exchange Server 5.5 Service Pack 3 (SP3) introduced the MIME Internet X.400 Enhanced Relay (MIXER) feature that RFC 2156 describes.

MIXER provides a method for mapping X.400 and RFC 822/MIME message formats, thereby improving communication between SMTP mail services (RFC 822/MIME and RFC 821-based protocols) and X.400 messaging systems. The MIXER functionality is most useful when you need to transport messages across a messaging backbone before Exchange eventually dispatches them to their final destination over an SMTP or X.400 gateway. MIXER ensures that the addresses that Exchange transports in the message header are correct when they reach the gateway.

MIXER Overview
The MIXER functionality lets you create an automatic gateway between X.400 and SMTP mail services without creating custom recipients to map SMTP and X.400 recipient addresses. The custom recipient strategy is valid when you have to map a finite number of addresses, but you can't use that strategy when the number of addresses is potentially infinite (e.g., when the destination domain is in the Internet).

MIXER supports three address-mapping types:

  • Encapsulation—To the message's X.400 site address, MIXER adds the Domain Defined Attribute (DDA) type RFC-822 and the destination SMTP address (i.e., DDA:rfc-822=SMTP Address as the value). Conversely, MIXER translates an X.400 address into an SMTP address.
  • Algorithmic mapping—MIXER automatically maps between X.400 address fields and SMTP. RFC 2156 describes in detail how this mapping process works.
  • MIXER Conformant Global Address Mapping (MCGAM)—Using a plaintext file (i.e.,, MIXER maps SMTP domain values to country, administrative management domain (ADMD), private management domain (PRMD), organization, and organizational unit (OU) X.400 address values (and vice versa). If you're mapping SMTP domains that the algorithmic method would translate to invalid X.400 addresses (e.g., the algorithmic method would map the SMTP domain *.com in the c=com field of an X.400 address, which is invalid because the country field doesn't have three letters), you can use the MCGAM method.

MIXER Components
To activate the Message Transfer Agent (MTA) MIXER feature on Exchange Server, you need the Exchange executable files included in SP3 (especially emsmta.exe and related DLLs), one Internet Mail Service (IMS) connector and one X.400 connector, two text files, and three new Registry values.

You must manually create the two text files (i.e., and in the MTA run directory (usually \exchsrvr\mtadata) to act as mapping tables for the MTA. In the file, you specify mapping between X.400 addresses and SMTP and vice versa. For example, if you want to map the SMTP domain to the X.400 address c=US; a= ; p=company, you must add the following strings in the file:$company.admd$ .c$US#,

and the reverse

#prmd$company.admd$ .c$

For each mapping set, you must add the appropriate mapping strings in this file. You must define algorithmic mappings for the SMTP domain that produces an invalid X.400 address when MIXER automatically maps the address. Moreover, you must consider how MIXER maps names (in the address) between SMTP and X.400 formats. The Microsoft article "XCON: User Name Mapping Issues When Implementing MTA MIXER" ( kb/articles/q239/0/07.asp) explains the limitations of the MIXER address-mapping scheme. Finally, if you have several MIXER servers in your organization, you must distribute the same file on these servers to ensure consistency.

In addition to the mappings in the file, the file contains Registry mappings that you apply for specific MTA MIXER servers. The Microsoft article "XCON: Exchange Server Message Transfer Agent MIXER Mapping Tables" ( explains how you use the file settings to override the mappings.

Mappings.out.txt is the output of the MTA MIXER for a given mcgam .in.txt file; the MTA creates this file when the MTA starts without errors. Mappings.out.txt contains the MIXER mapping table that the MTA is using. If the MTA starts successfully, you'll find the string New Tables Loaded at the beginning of this file.

You can modify three Registry values that affect mappings:

  • The AddressingNeverFails REG_DWORD value in the HKEY_LOCAL_MACHINE\SYSTEM\ CurrentControlSet\Services\MSExchangeIMC\Parameters Registry key—Setting this value to 1 instructs the IMS to pass the inbound message to the MTA when the destination address in the Global Address List (GAL) doesn't have an entry. If you don't set this value to 1, the IMS produces a nondelivery report (NDR). If the IMS finds a matching entry in the GAL, the IMS doesn't invoke MIXER.
  • The Strict Mixer Conformance REG_DWORD value in the HKEY_LOCAL_MACHINE\SYSTEM\ CurrentControlSet\Services\MSExchangeMTA\Parameters key—Setting this value to 1 instructs the MTA to use algorithmic mapping; setting the value to 0 directs the MTA to encapsulate the SMTP address in the X.400 site address as a DDA RFC-822 field. This mode requires an empty mapping table file; that is, if you use the encapsulation method instead of the algorithmic method, the MCGAM file must be empty.
  • The optional Reload Mixer Tables REG_DWORD value in the HKEY_LOCAL_MACHINE\SYSTEM\ CurrentControlSet\Services\MSExchangeMTA\Parameters key—Changing the Reload Mixer Tables value to any value instructs the MTA to recalculate the mapping table ( file) without restarting the MTA service.

Installing and Configuring MIXER
The most important aspect of the installation is planning the mapping scheme that your mail system architecture requires. With successful planning, configuring an Exchange server to act as an MTA MIXER is easy. Follow these steps:

  1. Set up an Exchange server with the X.400 and SMTP connectors, and install SP3.
  2. Create the and local
    .in.txt files in the MTA directory.
  3. Add the AddressingNeverFails Registry value, and set it to 1.
  4. To activate algorithmic mapping for addresses that you haven't specified in the file, add the Strict Mixer Conformance Registry value and set it to 1. For encapsulation, set the value to 0 and use empty mapping files.
  5. On the Routing tab in the Internet Mail Connector (IMC) Properties dialog box, set up as inbound all SMTP domains that you need to relay across the X.400 connector.
  6. Restart the MTA service.

Encapsulating an SMTP message on an X.400 connector causes message conversion, which is a CPU-consuming process. MIXER supports a special configuration that improves performance when you use the encapsulation method. This configuration instructs the MTA to encapsulate the SMTP message in X.400 without performing any message format conversion. To use this method, don't create any mapping file (i.e., instead of using a blank file) and don't create the Reload Mixer Table and Strict Mixer Conformance Registry keys. When MIXER encloses the SMTP message in the X.400 DDA field, the message will contain the DDA field SMTP: after address instead of the RFC-822:. This configuration improves performance as much as 22 percent.

In "XADM: Installing Exchange Service Message Transfer Agent MIXER" ( port/kb/articles/q238/5/06.asp), Microsoft recommends using MIXER on a server that has only one X.400 connector and one SMTP connector. Microsoft also recommends not using MIXER on a message journaling server or on a distribution list (DL) expansion server.

To improve system performance, remember that both X.400 and SMTP services generate a lot of disk I/O on the subsystem during message processing. Best practice is to use two RAID 0 volumes, and place the IMCDATA directory (IMS work areas) on one volume and the MTADATA directory on the other, possibly on two separate SCSI channels. Always use a dedicated Exchange server to reserve CPU for the message format conversion. If the X.400 connector uses an X.25 line, ensure that the X.25 link that you use has enough bandwidth for your environment: 256Kbps is a reasonable value in most environments.

Finally, install the post-SP3 Information Store (IS) patches that the Microsoft article "XADM: Exchange Server 5.5 Post-SP3 Information Store Fixes Available" ( .com/support/kb/articles/q248/8/38 .asp) cites. These patches fix crucial bugs related to the IS and to MTA behavior.

Using MIXER for Internet SMTP Connectivity
When I first discovered MIXER, I was interested in its primary application: improving interoperability between SMTP and X.400-based mail systems. Later, I realized that you can use MIXER features in other situations. Let's look at how you can use MIXER to create secure SMTP connectivity between the Internet and a private network that is completely closed to regular Internet access. In this situation, you need to break IP protocol continuity from the Internet to the internal network, which is a secure way to avoid potential malicious intrusions from the Internet. I use an X.25 WAN to connect the internal network to the demilitarized zone (DMZ—a computer host or small network that serves as a neutral zone between a company's private network and the outside public network), hosted by an ISP. In addition, I use firewalls to protect and filter access to the SMTP/ X.25 gateway (the MIXER server), both internally and externally. You need an internal firewall, especially if you don't completely trust the X.25 provider. Figure 1 shows a diagram of this system's architecture and server names.

Because the SMTP connector can't use an X.25 link, you must use two Exchange X.400 connector servers, one at each end of the X.25 link. Both external (GtwExt) and internal (GtwInt) Exchange servers have an IMS connector, which acts as a MIXER server. In this scenario, the MIXER on GtwExt uses the encapsulating mapping method to provide an SMTP/ X.400 gateway that automatically routes the SMTP/IP message to the X.400/X.25 connection, and vice versa. Here's the great benefit of using MIXER: Without it, you can't automatically route SMTP over X.400 connectors (and vice versa) without using custom recipients.

Finally, you place another Exchange IMS in the internal network (SMTPInt) and install it in the production Exchange site. You can use a mailbox server, but I suggest using a dedicated server.

Here are the most important steps for building the infrastructure, including the settings for each server involved. I assume that your SMTPExt server can receive from the Internet SMTP messages with the destination domain, which is the SMTP domain of the internal Exchange site.

The SMTPExt server is an SMTP server that forwards to the GtwExt's IP address all messages that have the destination domain and uses DNS resolution for foreign SMTP domains. This server doesn't have to be an Exchange server; it acts as a standard DMZ SMTP server.

You must configure the external firewall, FWExt, to allow SMTP communication between SMTPExt and GtwExt. Depending on your ISP's needs, you can configure the firewall to allow other communications between servers in the DMZ and the Internet.

GtwExt is the first MIXER server in the configuration. GtwExt encapsulates SMTP on the X.400 DDA field for inbound messages. Conversely, GtwExt unencapsulates SMTP from the X.400 field for outbound messages, forwarding these messages in SMTP format to the SMTPExt server. Because Exchange Server 5.5 runs only on a domain controller, you must set up the Windows NT server as a PDC. Furthermore, during the Exchange setup, you must create a new Exchange site that includes only the GtwExt server. Here's how you configure the server:

  1. Install and configure the X.25 card. Using Eicon cards simplifies the X.400 configuration because Exchange doesn't require third-party drivers to use them. The X.25 configuration is the most difficult part of this process because the X.25 configuration is based on several parameters that match the specific line used and the other server (GtwInt). Although I'm not an X.25 expert, I've easily configured the X.25 Eicon card. I used the WAN configuration (WAN Services) included with NT. You can also use the Eicon manufacturer's configuration utility, which Figure 2 shows. The sidebar "Configuring the X.25 Card" gives more information about this process.
  2. Install Exchange Server 5.5 with SP3, then configure the IMS connector to accept both inbound and outbound connections only from the SMTPExt server. To do this, go to the Internet Mail Service Properties dialog box. On the Connections tab, enter the SMTPExt's IP address in the Message Delivery text box. On the Address Space tab, set the Type to SMTP, the Address to *, the Cost to 1, and the Scope to Organization. These settings forward outgoing messages to the SMTPExt server. Finally, on the Routing tab, enter as the inbound SMTP domain, as Figure 3 shows.
  3. To improve security, set a local MTA password on this server in the MTA Properties dialog box. Remember this password because you must have it when you configure the X.400 connector on the GtwInt server.
  4. Before you set up the X.400 connector, you must install the X.25 transport stack. Using the Microsoft Exchange Administrator program, choose File, New Other, MTA Transport Stack and select the X.25 transport type. On the server's Properties page, which Figure 4 shows, configure the X.121 address with a sequence number (e.g., 1111). You use this setting when you configure the X.400 connector on the GtwInt server. Choose a display name, and enable the Eicon X.25 option if you're using an Eicon card or the Winsock X.25 option if you're using another card type. Choose the line type—Async phone line (Dial-up X.25) or Leased line—that corresponds with your X.25 line. Use the defaults for the other parameters.
  5. Install the X.400 connector with the X.25 stack, and use the connector's Properties pages to configure it. On the General tab, enter the Display name and Directory name. Enter the Remote MTA name of the remote server (i.e., GwtInt), and enter the Remote MTA password. In the transport stack Properties dialog box, enter the remote server's X.121 address. On the Advanced tab of the X.400 Properties dialog box, set the MTA conformance to 1998 normal mode, enable all X.400 link options, and enter IA5 in the X.400 bodypart used for message text text box. Finally, on the Address Space tab, set the Type to SMTP, the Address to, the Cost to 1, and the Scope to Organization.
  6. Configure MIXER to work with empty and files and with the following Registry values:
    AddressingNeverFails = 1
    Strict Mixer Conformance = 0
    Reload Mixer Tables = 0

GtwInt is the second MIXER server in the group. The configuration is similar to GtwExt; the primary difference is in the MTA routing table. GtwInt uses the IMS to route messages to and the X.400 connector to route messages to foreign SMTP domains. Like GtwExt, GtwInt is also a PDC NT server. Note these similarities and differences:

  • GtwInt's X.25 card settings are the same as GtwExt's settings. Follow the recommendations I made in Step 1 of the GtwExt configuration.
  • As in Step 2 of the GtwExt configuration, set up the IMS to accept both inbound and outbound connections and configure it to forward all messages to the SMTPInt server. Configure the IMS Address Space as follows: Set the Type to SMTP, the Address to, the Cost to 1, and the Scope to Organization. The IMS doesn't reroute incoming SMTP mail.
  • GtwInt's MTA requires the same password as GtwExt's MTA.
  • The X.25 stack has the same configuration as that on server GtwExt, except for the X.121 address, which you must configure with another sequence number (e.g., 2222).
  • When you configure the X.400 connector, the Name and Remote MTA Name parameters on the X.400 connector on the X.25 stack are the same as those of the remote server (i.e., GtwExt). Enter the remote MTA password you determined in Step 3. In the transport stack Properties dialog box, set the X.121 value of the remote server to the value you chose in Step 4 (i.e., 1111). The Advanced tab Properties are the same as those of the GtwExt server. Finally, configure the Address Space as follows: Set the Type to SMTP, the Address to *, the Cost to 1, and the Scope to Organization.
  • MIXER works with settings identical to GtwExt.

FWInt permits only SMTP between SMTPInt and GtwInt. SMTPInt is an Exchange server member of the internal Exchange site. Its SMTP connector receives incoming messages from GtwInt and forwards all messages to the GtwInt server's IP Address. So, you configure the Address Space as follows: Set the Type to SMTP, the Address to *, the Cost to 1, and the Scope to Organization. Optionally, you can add security restrictions that accept connections only from GtwInt.

I suggest that you always enable message-tracking options for all servers so that you can easily troubleshoot problems when they occur. Finally, I suggest that you back up MIXER servers once with an offline backup. You can use this backup whenever you need it, ensuring minimal downtime.

A New Internet Mail Possibility
MIXER lets you broaden your Internet mail capabilities. If your company needs to provide Internet mail to the user, the architecture I describe ensures a high level of security. MIXER might work for you.