Twenty years of MIME

The first MIME-formatted attachments were added to an email by Nathaniel Borenstein and sent on March 11 1992. One of the attachments was a photo, the other an audio recording of a barbershop quartet singing about the wonders of MIME.

The importance of MIME is that it defined a standard (eventually agreed by the IETF as RFC2045 in 1996) that email systems could support to ensure interoperability with other systems. At that time it was easy enough to circulate documents within the boundaries of an individual system as almost all email vendors had devised a mechanism to attach items, mostly word processing documents. Difficulties arose as soon as messages were directed to external recipients as they then had to navigate through one or more gateways to negotiate protocols used to communicate between different systems. Sending a message with an attachment from a system like IBM PROFS to DEC ALL-IN-1 was an adventure into itself and software such as SNADS gateways were created to enable communication to occur. However, these gateways were purpose-designed to handle the specific demands of certain email systems and were in no way general-purpose. In addition, they usually couldn’t handle a wide range of attachments.

MIME came along at a time when Internet messaging protocols such as SMTP and POP3 were being worked on to enable broad communication across the nascent Internet. The original definition for MIME included sixteen attachment types and reflected the narrow spectrum of attachments that were in general use at that time. Today there are over a thousand MIME attachment types defined.

The general-purpose nature of MIME became more useful as PC format types proliferated. Another aspect of MIME, its provision of a method to encode attachment data in a highly efficient manner, allowed email systems to send attachments without a huge overhead on the amount of data that had to be exchanged between systems.

The original version of Exchange didn’t include Internet connectivity. You had to buy and install a separate Internet Mail Connector (IMC) before Exchange could understand SMTP and MIME. This was understandable in that the foundation for the first generation of Exchange is based on the X.400 and X.500 ITU recommendations, which were seen as the best standards to use at the time when Microsoft started to work on the Exchange product. Soon after Exchange 4.0 was released, aided by a memo that recognized the importance of the Internet sent by a certain Bill Gates, a decision was made to ditch the separate IMC and instead provide the Internet Mail Service (IMS) as a core part of the product from Exchange 5.0 onwards. The move away from Exchange’s X.400 roots was completed only ten years later when the SMTP-centric Exchange 2007 shipped.

All in all, the Internet wouldn’t quite be the same as it is today without MIME. We certainly wouldn’t be able to cheerfully ship attachments of all types around with the ease that we can – even if it’s still not possible to drag and drop a complete DVD to a message. For its contribution to email, we should celebrate the 20th birthday of MIME and toast its enduring nature and the wisdom and foresight of its creators.

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