The following is designed to help Microsoft Exchange Server administrators compare the features, capabilities, and limitations of Site Connector and X.400 Connectors. This information may help administrators decide which connector will work best within their organisation.
Each customer environment is unique and proper testing and analysis of results should be performed to determine the best connector solution.

The Site Connector is relatively easy to use, especially when the Exchange Server computers involved are in the same Windows NT domain, or both servers trust the same master domain. Unfortunately, there are also fewer configuration options available for the Site Connector.
The Site Connector cannot be scheduled to send messages at selected time. A Site Connector is always active. Nor can message size limitations be configured on the Site Connector.

If message size limitations are desired over the Site Connector, you must restrict message size at the server's message transfer agent (MTA) level (or at the store level), which will also limit message size over all other connectors, and on message transfer between servers within the site using remote procedure call (RPC).

The difference in message delivery times between a Site Connector and an X.400 Connector (set to always) is negligible. The time for delivery depends on bandwidth, physical distance travelled, number of routers traversed, and so on, but all things being equal the difference in time is usually a matter of milliseconds.
As a general guideline, a Site Connector needs at least 56 Kbps of network bandwidth, available for Exchange Server communications only. It must be heavily stressed that the 56-Kbps transfer rate is a bare minimum for light mail flow, with little or no directory replication traffic.
More would be better, especially in a busy production environment. If there is a 56-Kbps dedicated line, which is also being used for file transfer, voice communications, or any other network activity, a Site Connector may not work well on a consistent basis. If thousands of messages/hour (including user mail, public folder, directory replication, and link monitor messages) are going across this link, 56 Kbps may not be enough to consistently establish and maintain a connection between servers.

Exchange Server MTAs communicating by Site Connectors use RPCs. RPCs run on top of network protocols such as TCP/IP (recommended). For additional information regarding RPC bandwidth requirements, please see the following articles in the Microsoft Knowledge Base: 177446 (http://support.microsoft.com/kb/177446/EN-US/) : How to Test Microsoft Remote Procedure Call Performance X.400 Connector

The Microsoft X.400 Connector requires one of three different MTA transport stacks (which must be installed and configured prior to installing the connector). The three choices are:

• TCP/IP. Installed by default with Microsoft Windows NT. This is the most common transport stack used by the X.400 Connector.
• TP4. Defined by the ITU (International Telecommunications Union, formerly known as the CCITT, or Commite Consultatif International de Telegraphie et Telephonie), TP4 provides connectionless network connectivity. It is often used on Ethernet networks. For additional information on installing the TP4 transport stack, please see the following articles in the Microsoft Knowledge Base: 148288ttp://support.microsoft.com/kb/148288/EN-US/) : XADM: Microsoft Exchange TP4 Transport Stack 152961 (http://support.microsoft.com/kb/152961/EN-US/) : How to Install TP4 Components on Windows NT 4.0
• TP0. Another ITU standard for network communications. TP0 requires X.25 hardware and software provided by an X.25 public carrier. Use of this transport stack is more common in Europe than in the United States. Eicon, SAT, and Cirel hardware/software is supported for use with Microsoft Exchange Server.

There are several advantages to using the X.400 Connector instead of the Site Connector.

• No trust relationships are required to connect two Microsoft MTAs with an X.400 Connector. Microsoft fully complies with the ITU's X.400 communications standards, which do not require Microsoft NT LAN Manager (NTLM) authentication. Additional security is not usually necessary, but can be added to the connection by adding a remote MTA password and/or
• Outgoing/Incoming Open System Interconnect (OSI) address information on the Transport Stack and the X.400 Connector property pages.

• Remotely Initiated. No queued mail will be sent until a remote MTA connects to this MTA. Both MTAs must have the two-way alternate option selected on the Advanced tab of the X.400 Connector for this to work, and only one of these connectors can be set to remotely initiated, or mail will not flow.
• Never. Mail will never flow over this connection. Can be useful for testing other routes.
• Always. The connector is scheduled to be active for consecutive 15- minute time segments. In essence, it is continuously active.
• Selected Times. This option schedules the connector to be active in one- hour or 15-minute time segments (see the Detail View option on the right side of the page).

A Site Connector requires more bandwidth than an X.400 Connector, and can only connect to other Exchange Server sites. Site Connectors also rely on Microsoft Windows NTLM authentication to work successfully. If these three conditions are met (bandwidth, Exchange Server, and NTLM), then a Site Connector will work fine.
There are many more configuration options available with the X.400 connector, including transport stacks, message size limitations, scheduled connection times, and connections to foreign MTAs. If the default values are used when setting up an X.400 Connector, the configuration is not difficult.

A Microsoft Exchange X.400 Connector can also be used to connect and transfer mail between non-Microsoft MTAs.

The information in this article applies to

• Microsoft Exchange Server 5.5 Standard Edition