The X.400 Connector and the Internet Mail Service (IMS) can both connect autonomous sites, but systems administrators have strong opinions about which connector to choose, based on their experience. Here are some pros and cons of each connector.

X.400
Advantages
Higher throughput than the IMS (about 30 percent faster because of the overhead the IMS incurs translating messages to and from the SMTP/MIME formats)

Tight integration with the Message Transfer Agent (MTA)

Reliable and robust performance

Ease of configuration and troubleshooting, if you're familiar with the X.400 recommendations and terminology

Disadvantages
Dependent on remote MTA name (if the name changes, the connector configuration must change)

Difficulty of configuration and troubleshooting, if you're not familiar with the X.400

IMS
Advantages
More options for security (Secure Sockets Layer—SSL—encryption)

Simplicity and flexibility in design (can use the same connector for Internet SMTP messaging and site connection, thereby reducing the number of connectors and routing information)

Host name and IP address independence (administrators can change the host server for the IMS site connector without coordinating with other sites, because naming is based on DNS mail exchange—MX—records

Ease of configuration and troubleshooting, if you're familiar with SMTP

Disadvantages
Higher overhead than X.400 because of message conversion (about 30 percent slower)

Difficulty of configuration and troubleshooting, if you're unfamiliar with SMTP

Not as reliable and robust as X.400

Based on my practical experience, I've found that the IMS's advantages make it a better choice than the X.400 for connecting independent autonomous sites. I've found that the performance, although not as good as the X.400's, is more than adequate. The IMS's other advantages (e.g., SSL security, MX-based naming, simplicity, flexibility) make the IMS easier to implement in the independent autonomous site. For more information about SSL security, see "Securing the Exchange Server 5.5 Internet Mail Service" (September 1998).