X.400 is a binary format, Internet Mail looks like text

Traditional arguments in this debate include:

• The binary format of X.400 makes it easy to include binary stuff without encoding it for transfer. Transferring binary data in Internet mail requires stuff like UUencode or Base64.
  On the one hand, this grows the data by up to 33%; on the other hand, encoded stuff like this has a good chance of surviving the vagaries of mail gateways, where binary data is notably prone to trigger "interesting" behaviour.
• The text format used in Internet Mail headings makes it trivially easy to show all present headings, no matter why they were added or by whom it is defined; in a binary format like X.400 headings, the most reasonable thing you can do with heading extensions that you don't understand is hide them.
• When you want to debug a problem with a text oriented protocol like SMTP, you can often simply telnet to the SMTP port on the troublesome system and type commands by hand; this is impossible using X.400. This makes the debugger's life easier, but also makes faking mail quite a bit easier.
• When something comes in that does not fit in with the specific syntax, an ASN.1 compiler will usually have to resort to general messages like "syntax error" (there are examples of products that return "syntax error" when a timezone value of +3000 is encountered); a text oriented protocol can display the unparseable value as text, which can be quite confusing to a naive user, but at least preserves the information.