Governing Standards Organizations
The International Telecommunications Union (ITU) developed X.400 recommendations, and those recommendations were first published in 1984 (red book). A revised version (blue book) was published in 1988. The ITU charges for X.400, and systems must be certified as X.400-compliant by the ITU. X.400 can run on many network protocol stacks.
SMTP was developed by the Internet Engineering Task Force (IETF) and was first published in Request for Comments (RFC) 822. SMTP was later defined on Internet Protocol (IP) and for use with the Domain Name System (DNS). These RFCs are available without charge and any e-mail system that wants to adhere to the standard can do so. Because of this, SMTP is widely accepted throughout the world.
Complexity
The X.400 messaging standards are published in two large books. The combined RFCs for SMTP are merely a few pages. This makes implementation of SMTP easier and less costly.
SMTP is also easier for users and administrators. X.400 addressing and routing is in the following format:
c=us;admd=microsoft;prmd=seattle;g=some;s=one
SMTP addressing in the following format is easier for users to remember:
some.one@microsoft.com
SMTP routing is easily understood if the administrator is familiar with IP and DNS.
Troubleshooting
X.400 is harder to troubleshoot because of X.400 complexity. SMTP troubleshooting is as simple as opening a telnet session to port 25 of an e-mail server and issuing SMTP commands. SMTP error messages are usually easy to decipher. You can test network connectivity with File Transfer Protocol (FTP), ping, Nslookup, and other basic utilities.
Overhead and Content Conversion (Message Size)
X.400 uses a native binary format known as Abstract Syntax Notation One (ASN.1). Because of this, every message needs to be converted into ASN.1, either by the messaging store or the message transfer agent that is responsible for transmitting over an X.400 connection.
By contrast, SMTP uses an ASCII (text-based) format. Because most e-mail messages that are sent are text or Hypertext Markup Language (HTML) messages, very little content conversion needs to be performed on messages. Additionally, most modern clients are Multipurpose Internet Mail Extensions (MIME) compliant, and the conversion is handled by the individual clients. In fact, many SMTP servers perform no conversion at all. For example, Exchange 2000 Server uses MIME as the native storage format. Only older Messaging Application Programming Interface (MAPI) clients need to convert from MIME. For Post Office Protocol version 3 (POP3) and Internet Message Access Protocol (IMAP) clients, the original content is preserved.
It is important to note that SMTP does require about a 30 percent increase in size for binary content, such as executable attachments, because this content has to be encoded by using uuencode or BinHex. SMTP is not designed to transport large file attachments, because FTP is the preferred method to transport large file attachments. However, in many environments, a 30 percent size increase is acceptable.
Reliability
Because there are more options for optimizing performance in low bandwidth situations in X.400, some claim that X.400 is better for those environments. However, SMTP is easier to troubleshoot when there are network problems. You can modify default SMTP time-out values on many systems. Also, SMTP performs very little connection management; connections are handled by the IP stack. Because there is less overhead for connections and less content because of message size, SMTP is a very reliable transport over high-latency, low-bandwidth networks. The very fact that SMTP was developed for the Internet attests to this. Generally, SMTP does not work if something is wrong with DNS or IP. These problems are also easy to troubleshoot, and are often indicative of other network problems, not just messaging problems.
Security
Neither X.400 nor SMTP is secure in its native format. Both have methods to secure either the transport or the underlying network stack. There are many conflicting standards for securing X.400. However, SMTP can use Secure Socket Layer Transport Layer Security (SSL/TLS), virtual private networks (VPNs), or IP security (IPSec), all of which are Internet standards and are easily implemented on most systems.