86 Mail [insert theme from Dragnet] The story you are about to read is true. The names have not been changed so as to finger the guilty. It was early evening, on a Monday. I was working the swing shift out of Engineering Computer Operations under the command of Richard Herndon. I don’t have a partner. While I was reading my e-mail that evening, I noticed that the load average on apple.com, our VAX-8650, had climbed way out of its normal range to just over 72. Upon investigation, I found that thousands of Internet hosts7 were trying to send us an error message. I also found 2,000+ copies of this error message already in our queue. I immediately shut down the sendmail daemon which was offering SMTP service on our VAX. I examined the error message, and reconstructed the following sequence of events: We have a large community of users who use QuickMail, a popular Macintosh based e-mail system from CE Software. In order to make it possible for these users to communicate with other users who have chosen to use other e-mail systems, ECO supports a QuickMail to Internet e-mail gateway. We use RFC822 Internet mail format, and RFC821 SMTP as our common intermediate r-mail standard, and we gateway everything that we can to that standard, to promote interop- erability. The gateway that we installed for this purpose is MAIL*LINK SMTP from Starnine Systems. This product is also known as GatorMail-Q from Cayman Systems. It does gateway duty for all of the 3,500 QuickMail users on the Apple Engineering Network. Many of our users subscribe, from QuickMail, to Internet mailing lists which are delivered to them through this gateway. One such 7Erik identifies these machines simply as “Internet hosts,” but you can bet your cookies that most of them were running Unix.

Apple Computer’s Mail Disaster of 1991 87 user, Mark E. Davis, is on the unicode@sun.com mailing list, to dis- cuss some alternatives to ASCII with the other members of that list. Sometime on Monday, he replied to a message that he received from the mailing list. He composed a one paragraph comment on the orig- inal message, and hit the “send” button. Somewhere in the process of that reply, either QuickMail or MAIL*LINK SMTP mangled the “To:” field of the message. The important part is that the “To:” field contained exactly one “” character, without a matching “” character. This minor point caused the massive devastation, because it interacted with a bug in sendmail. Note that this syntax error in the “To:” field has nothing whatsoever to do with the actual recipient list, which is handled separately, and which, in this case, was perfectly correct. The message made it out of the Apple Engineering Network, and over to Sun Microsystems, where it was exploded out to all the recip- ients of the unicode@sun.com mailing list. Sendmail, arguably the standard SMTP daemon and mailer for UNIX, doesn’t like “To:” fields which are constructed as described. What it does about this is the real problem: it sends an error message back to the sender of the message, AND delivers the original mes- sage onward to whatever specified destinations are listed in the recip- ient list. This is deadly. The effect was that every sendmail daemon on every host which touched the bad message sent an error message back to us about it. I have often dreaded the possibility that one day, every host on the Internet (all 400,000 of them8) would try to send us a message, all at once. On Monday, we got a taste of what that must be like. I don’t know how many people are on the unicode@sun.com mailing list, but I’ve heard from Postmasters in Sweden, Japan, Korea, Aus- 8There are now more than 2,000,000 hosts. —Eds.