Exim: The Mail Transfer Agent

Preface

Back in 1995, the central computing services at Cambridge University were running a variety of mail transfer agents, including Sendmail, Smail 3, and PP. Some years before, I had converted the systems whose mail I managed from Sendmail to Smail to make it easier to handle the special requirements of the early 1990s in UK academic networking during the transition from a private X.25-based network to the Internet. By 1995, the transition was complete, and it was time to move on.

Up to that time, the Internet had been a pretty friendly place, and there was little need to take many precautions against hostile acts. Most sites ran open mail relays, for example. It was clear, however, that this situation was changing and that new requirements were arising. I had done some modifications to the code of Smail, but by then it was eight-year-old code, written in prestandard C, and originally designed for use in a very different environment that involved a lot of support for UUCP. I therefore decided to see if I could build a new MTA from scratch, taking the basic philosophy of Smail and extending it, but leaving out the UUCP support, which was not needed in our environment. Because I wasn’t exactly sure what the outcome would be, I called it EXperimental Internet Mailer (Exim).

One of my colleagues in Computer Science got wind of what I was doing, begged for an evaluation copy, and promptly put it into service, even before I was running it on my hosts. He started telling others ...